And to bring up a likely reason: my impression is that H.264 and AAC still have active patents that apply to the major implementations, and those are not licensed gratis for all uses, and that applies more heavily to the encoding side. Raspberry Pi used to (and might still, I don't know) offer add-on license keys to unlock certain video codec support for what I assume was a similar reason. A lot of individual Linux desktop users just do their small-scale FFmpeg runs under the radar, but commercial software delivery is different.

With proprietary operating systems, the commercial entity behind the OS can negotiate for licensing. Some commercially-associated Linux distributions have efforts in similar directions, like Fedora for instance has some special secondary-repo stuff around H.264 support, but that's at minimum much more fragmentary as a target for an application developer concerned with the legalities.

Yep, but AAC is not, so you're still stuck needing to encode multiple times. This adds delay and also massive disk churn.