It probably depends on the music festival you're going to but there was one I used to go to every year

It was 3 or 4 days of day-drinking, eating like crap, staying up late, being outside in the sun on probably one of the hottest weeks of the year while probably not drinking enough water, and sleeping in tents on the ground.

And depending on how the festival is laid out, walking around the grounds from one stage to another, to different vendors and food stands, to your campsite and back, etc. can add up pretty quickly. The one I went to was pretty small and compact, but I still probably managed around 5-10 miles a day walking around, and you may be hauling around camp chairs, blankets, and coolers with you for a lot of that. And I'm not saying that that's a lot of walking, personally I can do that pretty easily, but it's more than a lot of people normally do.

unless you just aren’t taking care of yourself.

I think frankly, the tiring parts of music festivals for many people would very squarely fall into this category