Why would they? United are a self sufficient business. And as
@JPRouve has pointed out in his posts, the money was always there to be spent. So the question is why wasn't it spent untill SAF retired?
Well a theory I have is that SAF was mulling his retirement for quiet some time. I remember reading that he wanted to retire in 2012, but City winning it delayed the decision by 1 year. So my guess is he didn't want to spend big and not leave any financial resources for his successor. You don't make that decision in a month or two. It's clearly he was mulling the decision for at least 2 years.
Also SAF was being left behind with the times and "modern transfer game" as evidenced by the Hazard deal. We had the whole thing wrapped up and the terms were agreed with the player. The only sticking point was the "agent fees" which SAF refused to pay. As a result we know that happened next and how big the agent fees would get after his retirement.
The argument that we didn't have the money to spend is not true when you start examining our transfer business. We were supper close to agreeing a deal with Benzema until Real swooped in(remember the famous Owen interview where he was praying that the deal falls through so he can join United?), Hazard and a couple of others that have slipped my mind.
Also we couldn't compete with City wage wise, considering most of their players back then had a double wage. One on the books and the other one off shore.