literally just spouting hearsay, don't take this as fact -

My understanding is that slouching itself isn't all that bad. The issue is when you don't move around and shift your posture. If you slouch 24/7 it won't be good for you, but if you stand rigid as a plank all the time it won't be great either

& this part isn't even hearsay I'm just guessing, but I would guess it feels good for a similar reason that sugar tastes good. Even though we have an abundance of sugar (or time to slouch) nowadays, it was probably evolutionarily beneficial at one point to be encouraged to relax when possible.