there’s 2 factors:
- compatibility
- quality loss
AC3 is widely supported by players/receivers. AAC in its surround configuration is getting more support but is still far behind in my experience. So while basically all players with surround support will play AC3 5.1, not all of them will do the same for 6-channel AAC.
Both formats are lossy, which means they’re compressed by at least in parts cutting stuff away (e.g. frequencies outside the usual hearing bandwidth). They’re already compressed when you get them on your DVD/Blu-Ray. Quality won’t improve if you compress the compressed format – as many DVDs/Blu-Rays already include an AC3 track, you can re-use it as-is instead of transcoding it again.
So while your argument is not wrong (AAC is a more modern standard which can achieve the same quality at a lower bitrate), you need to consider that you’re not starting from a flawless source.
