I might be totally wrong, but it seems to me that:
A) In United's case, it seems to be all down to PSR issues. They simply have to churn as much of the dross on high wages, and, in turn, unfortunately sell the most profit-generative homegrowns as possible, otherwise they're stuck with a squad which is useless for their latest messiah.
B) I think in Arsenal's case, it's all about not wanting to spend in January what could be spent on better / better value in the off season unless a top-tier player were to be available. I'd imagine that we're starting to sail pretty close to the wind on the PSR bollocks too. Also getting payback for our lack of injuries last season. Fortuna balancing the scales....