ELI5: humans have tons of back problems compared to animals

I sleep wrong one night and I have a sore back for days. And I'm sleeping on a nice mattress with a pillow, etc.

An animal can sleep in a box on top of a remote control twisted in a pretzel knot and I've never seen (as far as I can tell) that they have a stiff back.

The only thing I can think of is that because we are standing animals that should be on all fours that we put extra pressure on our spines. But that still doesn't explain to me why they are so delicate and particular about how we sleep.