I only barely understand leaving for a headliner you're not interested in. The only time I've done it was for Flogging Molly a few years ago - which I attended solely to see Clutch - but I did stay for several FM songs and I admit they were fun. I just had something else to do afterward that I was more interested in.
Doing that for Mastodon, though, is incomprehensible to me. I know some folks don't appreciate them as much as I do, but it's not like they're awful, no matter what you like. How does someone like Gojira a ton and Kvelertak at least a little bit, but not like Mastodon at all? That just puzzles me.