A lot of people buy clothes for their looks with minimal weighting towards functionality (or rather that is the functionality). If you've got a body in reasonable nick then tight pants can look good.

The traditional solution to poor pockets is a purse or bag. Phones are interesting in that they can demand attention and that they are probably the most used item that people carry around with them. Thus people compromise their lines/comfort to actually use the faster accessibility of pockets. Probably explains the popularity of ridge wallets and key wallets too.

I don't wear tight jeans and I've come to loathe having to put my phone in my pants pocket. If you're walking around a lot, the feeling is just annoying, you can feel it pressing on your leg the whole time. When I sit down, the bunching feeling is even worse, so I immediately put the phone on the table.

This wasn't an issue when phones had 4.7"-5"-inch screens. Nowadays the phone goes into the cargo pocket if I'm wearing shorts, or back pocket if I'm walking.