Because you need to think of the room as another component in your system. Except this component is “unwanted” and, yet, inescapable regardless of how much “stuff” you have or don’t have in it. There are different ways to manage the room—speaker placement, acoustic treatments, a distributed bass array. One may have a nice system and think it sounds great but if one hasn’t tackled the room then one will never know who good it could be.
If you have a nice system why do you really need room treatments?
Yeah you may need an absorption panel if your room is completely open, ie. No rug or furniture, ie just lonely single chair. But if your system can't cut it in any room then it's a system problem and you should be able to discern a good system regardless of the room. Unless you put it on the roof of your apartment building but the Beatles seemed to have survived that effort
I think people go nuts with all this absorption acoustical room treatment stuff and it looks kind of awful. Once in a while you see a really cool looking diffuser panel and I would definitely want one. But to have a system that works really well without any of the acoustical panel distractions is a wonderful thing.