There is a pretty strong correlation between the size of a system's game library and hardware sales. For example, PS2 had the most games and it also sold the most hardware. Of course most games on any system are not worth playing. But the more games a system has, the more good games it has too. That's why it's simply better to have more games.
Switch is now on it's fourth year and it already has a ton of games. I'd expect that number to more than double before it's all done.