Arty farty, but there's nothing wrong with that.
Fuck all the scrooges who insist on Engineering being the only degree worth a damn.
Firstly, you underestimate how much say Ferguson has within United, there basically isn't a director above him. And secondly, or more importantly your second mistake, there is no second choice.
But, as I feel I haven't illuminated the situation enough, I'll spell it out: Pep does not fit the profile of a United manager. United will want a manager who can build, and importantly rebuild teams over the course of a decade, or longer. They want a manager who's a good coach and communicator, and to Ferguson that epitomises Moyes. And they want a manager with the same ethos as Ferguson, powerful wingers, lots of crosses, physical but fair.
And I'm leaving out the reasons why Pep wouldn't want to take the job, but I'm only half way through his book so I don't have the full picture. But his book starts with a paragraph that basically spells out how during his time at Barcelona, Pep realised he's not the sort of manager that Ferguson is.