Original reasons are hard to know without original author, but here’s a previous statement about that:
however it’s not clear if the current autoupdater will survive the needed migration from .NET Framework onto whatever .NET 5+ future is in store. I think there was a wish to find some third-party updater tool… Popular and well-resourced software such as web browsers seem to have updater services these days.
The heavily manual update approach is to stop the software, update somehow, and restart the software because just overwriting running executables (etc.) will likely work poorly. Must coordinate the activities.
Somebody else can maybe speak to macOS specifics. I don’t have it or know much about any specifics.
Replace auto updater #4542 has more discussion on this, and apparently Windows has the same issue.
As with most things, volunteers are too few, so feel free to help if this is an area that is of interest to you.
https://github.com/duplicati/duplicati/blob/master/Duplicati/Library/AutoUpdater/UpdaterManager.cs