Why even go to the concert?

I've been to a couple concerts this year and last year and noticed something kinda annoying (at least to me). I keep seeing but show up late into a concert and then leave early for whatever reason. For example, my sister took me and her brother to the blink-182 (openers Hot Milk & PVT) show and the two seats next to me were open for a while until halfway through PVT when these two ladies showed up. They didn't really look like they were at a rock/emo show but I'm not one to judge but were dead the entire time they were next to me. Which I thought was weird cause they were playing some of their best songs (in my opinion). Then they left halfway through blink. These seats weren't cheap either even though we were in one of the highest sections. So, I'm just left wondering, why even come to the show? This happened when I saw Avril Lavigne with my sister too! The group in front of us just sat on the grass and played on their phones before leaving early like. Why are you here if you're not gonna enjoy it? I just don't get it. /_ \

I just wanted to rant tho, also both shows were really cool and I'm so happy I was able to go! (The perks of having a sister with adult money)