My guess:  that's the nature of several decades of software development from programmers who come and go.

The update program works . . . most of the time.  But when and why it doesn't would require more programming than simply introducing a new update program to repair the things the old update program didn't get right.

Running repair updates every week automatically would probably take a long time (and put a strain on the servers if everybody is redownloading updates every week).  So the repair function is a "break glass in case of emergency" tool that only has to be run sometimes.