In addition to the steep learning curve, there is also the issue that most people do not have dedicated rooms where they can go buck wild with treatments. The vast majority of "home theaters" are really somebody's living room, where they have to contend with the constraints of WAF, aesthetics, furniture, etc. Remember that the folks on this thread buying a $2k+ receiver are a very small, niche subset of the A/V populace. Most people's jaws would drop at the dollar figures being thrown around here, myself included.
If you are spending that much moolah and have the luxury of being able to install good treatments, and the wherewithal to learn how to understand and measure acoustics, then by all means it should be a path you pursue; as you note it's hard to match the bang for the buck of improving room acoustics. Lots of people just throw more and more money at nicer speakers and electronics without addressing this.