This is a common audiophile misconception. In fact, a multi-channel receiver is the best choice for stereo, due to the fact that there is a room correction function (Dirac, Audyssey), which is key to getting a good sound.
And this is just one example.
Sorry, but that's not a fact, it's just marketing hype to get people to buy those types of receivers.
Receivers are versatile for home theater, but stereo amps are purposely built for music reproduction. Why you scream?
Stereo amps are designed specifically for two channel music reproduction. They focus all power and circuitry on music reproduction, they typically offer cleaner, more detailed sound with better dynamics then a multi-channel receiver. They often have higher‑quality components (DACs, power supply) dedicated to stereo use vs a multi-channel receiver.
A multi-channel receiver main focus is movie sound, and music is their secondary focus.
A person who wants something to listen to only music and doesn't care about a home theater sound system, will want a separate dedicated amp(s) for music. A lot of people have both, they turn on the stereo amp when they're wanting to listen to music and turn on the receiver when wanting to watch a movie. The problem arises when someone wants both without spending a lot of money for two different systems, those would probably want a multi-channel receiver if they're doing 50/50 for both types of sound, unless they tend to lean more towards audiophile music reproduction as being more important.
In my case I wanted audiophile quality music and didn't really care about movies. I have a separate amp/preamp system for music mainly, but I play movies through it, however I don't have a center speaker or rear speakers, I have two large floor standing speakers and a subwoofer, and when playing movies I get all the sound realism I need or want, my system actually sounds better than going to local movie theaters...but none of the theaters in my city have very good surround effects. Most movies don't even have much going on in the way of surround sound effects, only the science fiction genre have a lot going on in that area, and I watch very little sci-fi movies, and when I do it comes alive enough for me.