All bands have to start somewhere, whether that's playing a show of 20 people that's mostly friends and family, following music festivals like Warped Tour or opening up for a bigger band when nobody knows who they are.
I get it, you go to a headlining show for the headliner. You showed up 8 hours early to queue in line for the headliner, you paid to meet them, you buy all their merch, and you play on your phone during the openers in hopes that the time will pass by.
I've been to plenty of shows where I didn't pay attention to the opener. I'm guilty of wishing them off the stage, playing on my phone, the whole nine yards, so trust me when I say I understand when you say you didn't pay to see them.
As time went on though, I started to pay more attention to the openers. If you actually listen to their music and watch their stage presence, there's actually some really good stuff in there.
Some of the bands I love today were once openers who didn't have a large following like Waterparks, State Champs and AS IT IS. And if it makes you feel any better, I flopped SO HARD on State Champs when I first saw them three years ago. It wasn't until this year when "Living Proof" dropped that I realized what I had been missing.
They're putting their heart and soul into this whole music thing with the dream that they'll be the headliner one day. I promise you that it's not going to kill you to listen to them for 25-30 minutes. You may discover a new band to dive into. You may end up buying all of their discography, and maybe, just maybe, buy a ticket to their next show when they come back to your city (which could be a headliner!)
I've made it a thing to pay attention to at least one of the openers at every concert I go to, mostly because I thrive off of live music, but because I'm also always looking to expand my music library.
Pay attention to that opener next time. You may just find your next obsession.