Part of the reason for not allowing default apps to be set programmatically is that everyone else was abusive with this functionality. You’d install some 3rd party app and it would hijack your default settings to use their shit program. Or malware would do something like set a file association for .exe files so every program launch would relaunch their malware.

At least this way, only Microsoft is abusing the user and there is one set location to fix things. Not that they aren’t extremely presumptuous about it, but at least it only comes from the one vendor and not all of them.