My room was at a 5 or 6 for a while ... we were having guests and to make the rest of my house all at level 1, I tossed EVERYTHING into my not-that-big bedroom. And one of my friends opened the door and saw it! I was mortified.
It's at 4 right now... with any illness or trouble, I could easily see it growing into a bigger problem. Fortunately I'm healthy and happy... but not everyone is. Adding any illness (mental or physical) to an existing clutter issue is just a recipe for this sort of living situation.
For people who are used to living with clutter... it's different than if you just walk in someone's house and their place is a wreck. Because it's YOUR clutter. It's your mess. It grew gradually, and you just get used to each subsequent level. You get used to not using furniture - or that chair's job becomes to hold stuff. You get used to pushing stuff over, moving piles around, or sitting on the pile. You get used to stepping over or walking on things, and knowing the best path through/over the stuff. When I was a teen, my bed was often unusable due to the clothes on it (I was probably at 4-5 all the time then) and I would sleep in a pile on the floor. It wasn't a big deal to me. The stuff stops registering after a while. Which doesn't mean there isn't an underlying and all-pervasive sense of guilt, dread, and shame... just that if you're used to a pile of clothes being in that chair, you don't notice it anymore - unless someone from the outside (friends, neighbors, anyone) might see it.