Because this is Real Madrid. Look at their record, its a long term project, best team in the world. Major rebuild going on at United.
Good question. I would say a combination of
(1) The rot has been going on a long, long time
(2) British football vs European elite football; culture, climate, length of season, etc.
(3) Poor decision making at United.
While you kind of answer yourself here, I don't agree with your figures, or the level of rebuild needed. It's a work in progress. I would argue that the team that will compete successfully in England and in Europe has not yet been built. Fellani is a kind of manager's answer to this; tough and competitive and ready to give his all for the shirt. He played excellently yesterday. Your needless sideswipe at him (and indeed Lingard) doesn't add to your argument. The Glazers? Long old story, let's sit down at the fire with mad old uncle Roy and listen to horse stories. But we don't see too many green shirts around the club these days; and I am afraid that bird has flown. We might envy Real Madrid their ownership structure, sure. But the United owners have shown a willingness to invest; they look like perfect little angels when compared to some club owners these says.