Because the best third party support and games are all on MS, Sony and PC. So the natural response is to defend the home ground.
Now if Switch (or their next system) suddenly got just as many good third party games as other platforms, suddenly all the R*, EA, Call of Duty, UBI games etc..... would suddenly be great and a good boost to the game library.
Nintendo dominated third party console devs during NES and SNES and everyone loved it. I don't remember any Nintendo gamers back then (including me) complaining about having awesome Konami and Capcom games while Genesis had worse ports, and SMS got zero ports.
Gamers seemed to love Mario, Zelda, but also EA Sports (when their white label sports games became a hit), along with all the awesome SE games, and any other third party games.