Because anything put on a screen makes it look like shit to me. I didn't pay that much money on a top of the line phone only to make it look like a low-end one.
I don't understand people with screen protectors. Unless they plan on reselling it.
BTW, I think people with iPhone X but with a screen protector are making a fundamental signaling mistake: you show to people that you can't really afford the phone.
I use a leather case and plastic screen protector (7 Plus) not because I care about resale value, but rather because I don't want to coddle my electronics; I want them to be fairly durable. The way I see it, the dividend from the trend of maximally thin phones is the ability to put them in a robust leather case without them becoming too bulky.
My anecdotal experience has been that a plastic screen protector greatly reduces the likelihood of screen breakage, presumably because the soft plastic can absorb some amount of impact force. I've never ever broken a screen, and I drop my phone fairly often.
I also prefer the finger-feel of the Belkin plastic screen protector. And I enjoy replacing the protector every 6 months or so and walking out of the store with an absolutely perfect, scratch-free screen.
(I tried the Belkin glass screen protector once; it cracked within a month. And I disliked the finger-feel.)
Why? At least use a glass screen protector if it's the feel.