I agree that there are quite a lot of people who want 2.1 audio only.
But are there enough to make special dedicated 2.1 audio-only systems for them, that are cost-effective, compared to a 5.1 A/V system?
Just as there aren't a load of cheap 1-channel amplifiers out there in the mass market, because everyone wants at least 2-channel. (1-channel amplifiers exist, but they're boutique, not cheap.)
2.1 is kind of in the same situation. Sure, you can bemoan that by buying an AVR you're "wasting money" on bits you're not using that could have gone into, say, better quality amps/DACs for the 2 channels. Except that's more than offset by the mass-production price gain of sharing the gear with the multichannel crowd. (Mono 1-speaker people are also best off buying a 2-channel system...).
The two mass markets are "5-or-more-channel+sub" (mostly home cinema) and "only-2-speakers" (mostly music). Everyone else just has to go along with it.
(And they're not even mass-market anyway, are they? There's probably more for silly little USB speakers and headphones... They're the two "hi-fi" mass markets, at least)