People will drop thousands on a cable, $10K on an amp, or turntable, but I don't see anywhere near that spend on acoustics in most cases.
There's a lot of reasons, and you already listed many of them. But like so many other things you can get the lion's share of the results for next to nothing and then spend a fortune to eke out the last few percent.
Acoustics in my listening room are so much better than the rest of the house everyone notices the minute they walk in the room. Yet the most obvious improvement is from some ordinary Owens Corning 703 acoustic panels that cost probably only about $100 altogether. The fabric covering them cost way more than the actual acoustic material! This was all done like the rest of the system by a process of trial and error over a period of time. https://systems.audiogon.com/systems/8367
I know from experience this simply does not have a great deal of appeal. The vast majority of guys would rather pay big money to some "professional" with "expertise" who will tell them what to do. The idea of walking around clapping your hands and listening, or even listening to music, moving panels, listening some more, these things are too much "work". Work is not cool. Spending money is cool.
Also the most effective acoustic treatment in the room is hard to understand and almost invisible. Its the Synergistic Research HFT on the speakers and walls. Look close, they are only about 1/3" diameter. Bling factor near zero. Audiophiles like bling. These ain't it.
Oh and audiophiles love narrative. Stories. Dither is not a very good story.
My room could really benefit from some diffusion panels. Something that wasn't apparent years ago but now as the system improves its becoming more apparent. I know what to do. Know exactly what I want, and how to make it.
Unfortunately it involves work. Which proves my point.