Who said you can't????
Just kidding. Know your question
Well, Jim put a good point across. Some other important points are: Bass Management and priority of DAC/DSPs.
AVRs tend to do everything 'the cinema way'. The sound stage is presented with low power positional satelites. Sub is extremely important (never saw an HT without a sub). Low end grunt is handled by a sub dedicated to the duty. In brief, an AVR, out of the box, is optimized for home cinema usage.
A stereo receiver is, and can only be, for two channels. Bass management is not a principle. Amp amplifies the whole stereo signal without passing them through a filter. (exception being recent amps with a sub out). Result? Fullness of the sound.
That said, an AVR is not 'incapable' of stereo duty. A good AVR can beat an average stereo receiver by a huge margin if configured properly. I have myself used an AVR for stereo duty for a long time. And the sound was better controlled than my previous stereo amp.
The point is: Out of the box, an AVR is not suitable for stereo duty. But a really good AVR can be configured for stereo duty and it will not be far behind. Yes, generally when people buy an AVR, they want it to be a dual use system, which is not recommendable.
Buy a good AVR, disable all but front channels, set speaker size/type to large, disable/set the bass threshold to the lowest AVR allows, buy full range towers in stead of bookshelves+sub, use pure direct if available and you can get very good sound from an AVR.