The main problem with downloading APKs from the GitHub releases page, is that it doesn't come with a low-friction way of distributing updates. This is especially important for unofficial client apps like NewPipe, which basically become bricked at a moment's notice, and updating within a day or two is important to maintain continuity of service.

Not to mention that an app may be hosted on a different platform, perhaps one that doesn't do auto-builds. Or, as was already pointed out, builds do exist but contain telemetry or secret code. In that case, a trusted intermediary (which I trust heck of a lot more than Google) that pulls and compiles the code and then distributes it, is a strictly value-added proposition.