From generation 2 all the way to generation 6 I never owned more than one home console at a time. However, I had owned a PC from 1993 and onward, and I did a lot of PC gaming up until generation 7. So I think I usually had two platforms, but my second platform was really the PC.
Somewhere around generation 7, I decided I was through with PC gaming, so I got a PS3. It had almost no games. So I bought a Wii and it had a lot of fun games. By the time I was tired of my Wii, the PS3 had a lot of fun games. Then I got a Wii U. It was ok, but not nearly as good as the Wii. Then I got a PS4. It was the most disappointing home console I've had yet. Somewhere in there I also got a 3DS which was the most disappointing handheld console I've had yet.
Then I got a Switch at launch. It had a few dry spells the first year, but now the games I want are coming out faster than I can play them. That is a good problem to have. I don't foresee myself buying a new console anytime soon, because there are still tons of games I want on the Switch.
In the end I think that is what it comes down to, "Do I have enough games to play on my current consoles?" If not, then people will get another console. For some people 1 console will be more than enough. For others, they might need all 3 and a PC too. It just depends on if people feel they have enough games for themselves to play.