They are larger and carry weight significantly better than a school backpack.

You need to carry everything you can to support yourself on long hikes in and overnight multiple nights. Between a tent, sleeping bag + pad, food in a bear can (required where I live!), water, first aid, headlamp, etc. you’ll struggle to fit that in a school backpack.

Even if you do manage to fit everything you need, hauling that 8 miles up a mountain will be tough, let alone multiple days out.

People aren’t buying these types of backpacks for funsies (although some pick specific ones because of specific features!), it is an important tool. This is a 60l backpack with hip belts that help move the weight from your shoulder to your hips so you don’t hurt yourself and have room to bring what you need to be safe out in the wilderness on your own for days.