For instance The Hitman's Bodyguard was just released without support for 5.1 (English (Stereo) only). I also recall that John Wick 2 was released the same way along with other movies. So if you buy the movie and watch it that day, you are getting only stereo, but if you wait then at some point the movie is in 5.1.
Its bad enough that we can't upgrade up to higher quality but to release in stereo seems woefully behind the times.
It's all down to what the studio provides.
It's been my experience with rentals that the SD versions typically come with regular stereo soundtracks and the HD versions have 5.1 Dolby Digital (though sometimes only stereo).
Can't comment on purchased movies as I simply wouldn't purchase a movie from a streaming source. Rental is fine - very convenient and 'good enough' quality for the price - but if I pay to own it, I want to own a physical copy in the highest fidelity possible.
Currently there aren't any streaming services that can match optical media if you have a 4K TV. Vudu does 4K movies, but they only support Dolby Vision HDR. Google doesn't even support HDR but doesn't offer anything with Dolby Atmos. Also, I think it's crazy that a UHD disc costs around $30 (less with sale), but a 4K digital is the same price.
UHD disks support Atmos, all HDR, 4K, and I get a standard BD and digital copy with them. Win/Win.