Why would you limit yourself to first party titles?
Because that was the mindset that people basically was forced to use on the Wii U (after the launch months, very few larger games arrived that weren't 1st party), and they reapply that mindset now?
There are many good opportunities to limit the software selection for people interested in that:
* I only buy first party games.
* I only buy games on sale.
* I don't buy 3rd party games that are on other platforms.
* I don't like indies.
* I only play docked/undocked.
* I only play long/short games.
* I only play games with good HD rumble.
* I only buy physical releases.
* I only buy digital, but the storage requirements are too big.
* I don't have time to research the Switch software catalog. Over 1,100 games, and 106% of them are shovelware.
* There is nothing that interest me.
* I don't buy this new game, because it works differently than the previous entry.
* I don't buy remakes.
* I won't double dip.
* I only want virtual console games.
* I don't like 8bit retro games.
* I don't like Mario.
* And so on.