[Bjartr]

Based on reading a lot of The Old New Thing blog by MS veteran Rayond Chen, I think there's a pretty straightforward reason:

A user could accidentally do it and end up with a 'broken' menu they don't know how to fix, and Windows being 'broken' in that way is Windows' fault from the perspective of such a user.

This sort of thing can and does cause a support burden, which is an expensive tradeoff. So rather than it being a built in capability, a user would need to manipulate the registry or use a third-party program to do it for them.

At least, that's the reasoning that would've come up at MS when adding such a feature was suggested internally (and it certainly has been)

[causality0]

Seems like you know a lot on the subject, so I'll ask you about another missing obvious feature: why's there never been an option to auto-expand the task bar with the number of open windows? It seems unquestionable that if you have three windows open you need a single-row task bar and if you have twelve windows open you need a two-row task bar yet if you want one you have to manually unlock it and then manually expand it.