how do people sit in bleachers without their back hurting?
Bleachers have no back support. If I hunch over, my lower back hurts. If I sit up straight, my lower back is fine but my shoulders and upper back hurt.
The true bleachers experience is all about getting burnt/frozen when sitting down, then having to endure the pain of zero padding for however long you have the displeasure of sitting there.
There is nothing you can do, bleachers are designed for maximum cheapness without posing a huge safety liability, and have zero dollars put into comfort during development. Literally minmaxed to optimize for cost.
I think this is a case of weak muscles. Don't get me wrong it doesn't surprise me given how many people sit on chairs and couches all day. But yeah. Start with air squats.
Your posture muscles are mostly around the thoracic spine but are quite complicated. The weight of your head, and shape of your torso have a big impact on how much strain is placed on the muscles that enable posture. The heavier you are up high, the harder posture will likely be. Most people don't really notice it very much.
In other words, it is mostly muscular endurance and weight.
I see no one mention sitting on your sitting bones. People have a tendency to collapse their spine at the base, and then rotate their pelvis backwards to slouch, even if their upper body is straight.
If you sit aligned you can feel the bony part of your butt touch the seat, which although uncomfortable at first, makes sitting longer more tolerable after time.
At university we would have a lot of classes without chairs, so we'd sit on the floor a lot, and the above would help.
There are bleacher seats with back support, but they are cumbersome to carry and if you stand up, you can knock them onto the people behind you. Also, they're a bit anti-social in a sold out event where everyone's squeezed in.
I've found inflatable bleacher cushions to be a good option. Some store very small in a pouch, then you blow them up at the game. If you tip them off the seat, nobody gets hurt. No back support, but they make sitting a lot more comfortable.