I was a late smartphone adopter (bought my first in 2015) and it took quite a while before I realized lots of people are using cases and screen protectors. I couldn't understand why, I thought they just fell for the marketing of useless accessories. But when I started asking people and reading online discussions about it, I learned that many, if not most people frequently drop their phone and get scratches on the display from, I don't know, putting it facedown on rough surfaces or storing it in pockets with sand or something? (Keys are not hard enough to scratch glass screens, right?)
I guess I'm just a bit more delicate with my phones, I've never dropped one in over 10 years of smartphone use and never got visible scratches while I've never used a screen protector. And I hate how clunky and ugly phones become with a case, completely destroys the elegance of their design.
I've tried cases to counter the slipperiness of phones, but I couldn't stand them very long. And quite soon, I learn how to grip a phone right, and they also seem to get less slippery with prolonged use somehow—but that might just be me getting used to it.
I guess there's three types of people: those who genuinely need the extra protection, those who think they do because they see everyone else use it, and those who know they don't.