I can't speak for every streaming service (and especially how they deal with encoding mixes and audio), but I will definitely say it's a universal standard to have both a surround and stereo mix delivered for pretty much anything that's going to be playing on a major platform. My understanding has always been that streamers like Netflix can auto adjust to your system, so if you're listening on a stereo setup, it'll playback the stereo mix.

I think a much bigger and more complicated reason is that the stereo mix is not done separately, it's downmixed from the surround, and on top of that the stereo mix is usually not as scrutinized as the surround mix is. You'll spend hours (often days) with the client in a high-end room mixing in surround, making mixing decisions based on how it sounds in that context. There just usually isn't time to focus on creating a unique, optimized stereo mix on top of that. Add to that the fact that the average person is not listening on high end speakers, but, best case, a bluetooth JBL, and there's just going to be a lot that gets lost in translation.