There's at least four other possible reasons which have been speculated on in this thread. Fact is, no one knows. But it is the pressing question.
I'm sure that you have noticed a marked deterioration in the quality of discussion in the last couple of hours. It's great isn't it?
Personally, I don't think the fact that the Glazers haven't taken money from the club represents the "pressing question".
You are simply doing what everyone else has done for the last ten months. We have all been brainwashed into thinking that it is a given and now it isn't there, we want to know why not!
Maybe the other had it all wrong from the very beginning?
The assumption is that the Glazers have no money of their own, the assumption is that their other business interests have been losing money hand over fist.
The assumption is that without Manchester United, they have no hope of paying down those PIKs.
What if all those assumptions are merely... well... wrong?
What if the Glazers simply set up that £70million carve out thing as a safety measure for if their REAL plan didn't quite work out?
What if the Glazers gave themselves the option to take out a £25million dividend in case they had a bit of a cashflow problem at some point?
What if they never actually specifically planned to take any of it and are happy to see the value of United grow?
Presumably, they were all living pretty comfortably before 2005 and have continued to do so ever since.