Why is it that despite having a PS4 Pro and many of the games people keep bringing up, I still play and buy more games on my Switch?

Have you even read through the thread?

Of course everyone has their opinions and preferences on what system is their "primary" console. Is the Switch viable to be a primary? Well like I stated it's about your preference and what you like. The Switch's biggest advantage's are the Nintendo games and portability. That's it. Now if you want the argument to be why the PS4, Xbox One, and PC are much better options for a primary gaming device, then I'll list a few. Yes I know you said "primary gaming device" but you can't ignore what your gaming device offers other than just gaming.

Now the PS4, Xbox One, and especially PC get the "bigger" games that the Switch will not get. Also, games that do come to the Switch that are on other platforms are the worst versions that have low resolution, low texture quality, 30 fps, etc. All of that is what you get in favor for the portability. Now the biggest one in my opinion is the frame rate. Games are better when they are at a higher frame rate so when you go from 30fps to 60fps the game not only is faster, but more responsive to your commands. Playing DOOM at 30fps isn't good, it's playable but playing at 60fps + makes the game much better especially with a mouse an keyboard. The motion controls are a really good addition, however it's still 30fps and you can use the Pro Controller on Steam using the motion controls at 60fps+. 

The PS4 and Xbox One have way more uses than just gaming.  The Switch only has Youtube and Hulu, however these systems offer way more apps such as Twitch, Netflix, etc. but the biggest one is a web browser. You have cross game voice chat whereas the Switch doesn't barely has any games you can use voice chat. The PS4 and Xbox One as stated before will run your games better than the Switch, and will get the "bigger" game releases. 

Now the PC, oh man the PC. Way WAY more benefits than a console. The best versions of games, much bigger library of games, cheaper games, free online, more genres, multiple control options, etc. Now the biggest benefit to PC is it's a PC. You can do way more than just gaming. 

I agree about the PC. Objectively no gaming device currently can beat the library of the PC. Maybe ... maybe ... in the future mobile games take over the lead.

But I disagree that Xbox One especially gets the "bigger" games that the Switch will not get. Why then the Switch has the bigger software sales compared to the Xbox One? Either it has bigger games or more games.

Because the ps4 has far better third party support while exclusives are equally good

Take a look at the gameaward nomination and see which console has the better game support:

GAME OF THE YEAR: 5/5 PS4 1/5 switch

BEST ONGOING GAME: 5/5 PS4 1/5 switch

BEST GAME DIRECTION: 5/5 PS4 0/5 switch

BEST NARRATIVE:  5/5 PS4 0/5 switch

BEST ART DIRECTION:  3/5 PS4 1/5 switch

BEST SCORE/MUSIC: 4/6 PS4 2/6 switch

BEST AUDIO DESIGN: 4/5 PS4 0/5 switch

BEST PERFORMANCE: 5/5 PS4 0/5 switch

GAMES FOR IMPACT: 4/5 PS4 2/5 switch 

BEST INDEPENDENT GAME:  2/5 PS4 4/5 switch

BEST ACTION GAME: 5/5 PS4 2/5 switch

BEST ACTION/ADVENTURE GAME: 5/5 PS4  0/5 switch

...

What was game of the year last year?

Switch tends to get the old school games or games based on old school genres like Axiom Verge, Dragon Quest Builders and Octopath Traveler.  It's also the best console for local multiplayer.  Like I would never buy Puyo Puyo Tetris on a PS4, because I already know I'm going to spend money to get extra Switch controllers for games like Mario Kart and Smash.  So that really makes it the go to console for local multiplayer.  So, yeah Switch suits my gaming tastes really well.  And on multiplat games, I prefer Switch because the games I like usually don't have cutting edge graphics anyway.  Switch hardware is more than enough graphically and it's portable.

On the other hand I have a PS4, but have been just so disappointed with it.  So far the only exclusive I really like is the Spider-Man game which I am playing right now.  Everything else has been disappointing and slim pickings.  Assuming Sony continues this trajectory, then I have no interest in a PS5, even though I've had PS1 - 4.

So, obviously it depends on taste, but there are a lot of people out there who have a taste for Switch games.

..judging by the responses I guess you are right. Some of the responses I'm seeing really raises some questions for me

Imagine missing out on stuff like TLoU and god of war, or gta5 and RDR2, thats why it would never be most peoples only console. A great Zelda game every 5 year just isnt enough.

No worse than missing out on Zelda, Mario Odyssey, Pokemon, Xenoblade, or Smash in my book...and I hate GTAV and have no hype for TLoU2 BTW.

There is more to it than library alone.  Read the whole discussion as this has already been addressed more than once.