While it's obvious that MS would have a reason, I have no clue what that could be. The only thing I can think of is that people seem to use Google rather than their app's help functionality even if the latter would do a better job.
The Office team, however, seems to operate more or less independently from the Windows team. The ribbon, for example, was first unveiled around the time Windows Vista was released, but Vista had no ribbon anywhere. Windows 7 added the ribbon to a few random apps, like MS Paint. Windows 8 added it to a few more random apps, like File Explorer. But they don't seem to be in any hurry to unify the look and feel of their own flagship products.
Well, what else could we expect from a company that thinks it's OK to slap two completely different UIs (Metro and desktop) on the same OS? Microsoft UI is schizophrenic.