I think it's a combination of trying to appeal to the growing niche of tech chasers, while also turning video games into a service. A model more similar to smartphones is much likelier to be profitable than the old model.

It's not a new thing either. Turbo Duo. SEGA Wonder Mega. PS2 later models got a slight CPU speed increase. Game Boy went color. GBA SP added backlight. PSP 2000 had upgraded specs. New 3DS also upgraded specs from better CPU to more ram. PS4 Pro and Scorpio are the largest upgrades but still it's not a new practice.

That's an awfully complicated way to spell "money".

I'd say its 4k TVs more so than PC. If they really wanted to compete with PC, they would have stuck to 1080p and choose a cpu that could do 60fps instead of this consistent move to 4k.

PC hasn't been about 4k gaming at all. The aren't even that many 4k monitors and the ones that are 4k aren't even that good with a few exceptions. Yet both Ps4 Pro and Scorpio has been spewing out 4k left and right... And 1080p enhancements for the Pro has been more or less been optional. Even sony's own games such as UC4 and the Last of Us Remastered bearly even does anything to enhance those games...

It all comes down to money. A combination of double dipping, a push in 4K TV sales/relevance (more at Sony's end) and having the more appealing powerful console so people choose it over completion.

Agreed. There's certainly an overlap between the PC market and the console tech enthusiast market, but you would be talking about a fraction of a fraction.

In reality, PS and Xbox are casual products, the majority of its buyers see it as a FIFA/Madden/Just Dance/Netflix box, and couldn't care less about resolution, framerates and such.

It has more to do with selling UHD TV's than anything else.

But MS doesn't sell TV!! Well, the Scorpio is more like an "apology" console, after all the initial drama of the XB1 early months and the resolution gate, they are desperate to recover some of the ground lost to PS in that regard. 

There is no single reason. But Money, Competition, 4K and PC play an important role, in that order.

Of course the main reason are PCs.
The former console market is getting eroded by smart devices on the lower-end and by PCs on the higher-end.
Sony though has better chances to sustain their position due to 1st party studios and exclusivity deals.

The whole electronics industry has changed. Consoles are no longer in an isolated corner of the market. They are squaring off against competition from every corner.

In the past the consoles was the most logical choice for DVD Playback, and streaming services. Now that front is uder attack from Smart TVs, Set Top Boxes, Cable Boxes, and Streaming Sticks that use a smartphone to grab media.

They are also not the only player in town for games on the TV anymore. Amazon Fire TV has games, not great, but games. Steam is becoming more possible on TV with small form factor PCs, and Steam link. There is also the Nvidia Sheild which can do both media, local games and streaming.

Non of these competitors sit on tech for 5, 6, or 7 years. There are revisions to all of the competitive devices on a yearly bases. They add better wireless connectivity, lower power consumption, the latest video and audio processing, faster CPU/GPU processing, more RAM, more Storage, and continually improve the user experience.

Sony, Microsoft, and Nintendo are moving to scalable generations because they have to. The New 3DS, PS4 Pro, and The Scorpio are a must for consoles to stay competitive with the massive competition they face on every front.

Beyond the above, there is also the fact that tech growth has slowed and we have reached the point of massive diminishing returns. The increase in technology over a 5 - 7 year period in no longer enough to justify a full generational change in the way games are made. In 2020 with 7nm tech, we will only be able to achive a leap in tech, that is equivalent to half of the leap we have seen in every generation since Sony entered the market. It won't be until ~2023 with 3nm fabrication that we will have a leap in tech from the PS4 and XBO that can justify a new generation, and cut from the PS4 and XBO hardware.

For all the reasons above I fully expect Sony, Microsoft, and Nintendo to have yearly revisions of their hardware, with notable revisions every 2 - 4 year depending on how the fabrication process matures. I don't see them cutting ties and moving what could be considered a new generation any quicker than once every 10 years.

I fully expect one more revision to the PS4 and XBO platforms before the bar is rest. I expect to see mobile counterparts to both PlayStation and Xbox either under the brand or via Surface and Xperia. I expect to see Nintendo move to a yearly revision model that is almost identical to Phones, especially with their partenership with Nvidia, and the yearly improvements made to the Tegra processors.

This is not double dipping, it is not just about new TVs, nor is it about PS4 and XBO being to weak. It is about changed market, and trying to stay relevant. On the consumer side the choice is yours. Buy a PS or XB day one, and ride it out for 10 - 12 years, upgrade every 6 years, every 3 years, or every year.The great thing is, the choice is there for you to make.

Personally, I will be upgrading my PlayStation ecosystem every year, and will do the same with a Phone, Tablet/Laptop, and Handheld versions if they arrive. On the XB and Nin side, I will be getting the Switch and the Scorpio, and will upgrade again after either significant features are added, or a generational separation occurs. Thanks to this I will no longer be buying into Windows, iOS, or Android devices in the future.

Make money from double dipping.

These companies can't afford anymore to spend $600 to make a console and sell it for $600 so the specs can hold up and in the hope that everything goes smooth until the next gen starts 6-8 years later. If a major disaster or crash happens during that time, and people can no longer afford the $600 console at launch, or $500 price drop years later, these companies end up in major financial trouble.

By selling a console around $400 at launch, and actually making a small profit, while they are still at risk, that risk is greatly minimized by the console being $400 instead of $600. That just means more money left in the companies bank for an emergency.

This also allows the companies to gain more profits overall, and allows them to sell another mid gen refresh, making even more money, due to the fact that the hardware becomes outdated faster now due to the cheaper launch price and lower specs. All because they are playing it safe. The fact that it used to be normal to build things to last and keep them for extended periods of time, yet today things are built to break, and people are expected to, and some like to, buy updated things more often, makes this type of console cycle easier to swallow.

The rest of the points all fall into place, and are all byproducts of this, but making money is always #1. I would say 4k tv's #2 and PC #3.