I've been thinking about these sort of issues for some time.
In my opinion... Big club from big leagues have started exerting a sort of oligopoly regarding the best players around that there really isn't much room for youngsters in big teams, so it's natural that only the very few once-in-every-ten-years outstanding talents will have a place in the first team on the short term. Some might be happy just being a reliable back-up option, others wont.
Imagine a young talented RB at Real. He could look at the squad and see that he might take Arbeloa's spot soon because the later isn't up to the standards expected. That's his chance. But at the same time if Real is looking to sign one of the top 5 RB's in the world to cover that gap than the youngster will never have a chance. And that's what happening, things have changed quite a bit in the past ten/twenty years. In those times there were the outstanding players, surrounded by competent players who weren't necessarily among the top-10 in the world in their positions. But the standard has increased at this sort of clubs. You look at the first XI of a team like Bayern and Barcelona and you expect each and every one of them to be among the top-10 in the world in their position. Otherwise they're not good enough, and it has to be solved in the next transfer window. They're not going to wait for a player from the U-teams to grow and gain that spot.
You look at a talent like Bruma and he can very well rest assured that he'll be in the first XI of Sporting next season, having just turned 18. He wouldn't have that chance at United or Real. His natural progression implies to gain exposition and maturity at Sporting and then switch to a big club when he's good enough to realistically fight for a first-team space.
People seem to expect everything from clubs nowadays. If a club like Porto bought a South American outstanding talent for 5m than you wonder why United doesn't have a similar scouting work? Well, perhaps it may just be the case that they're not interested, because there's not much room for these talents to grow at United anymore. This implies giving them first-team highly-competitive football in Europe. That's a risk Porto can afford because we don't have anyone better than him in the first team, the same reasoning wouldn't apply to Chelsea or United. Same goes for youth players.
If 1.5m for Mats Daehli covers the costs of his football formation than your academy is at a gain already. You had the chance to bring up a potential first team player, and when that chance doesn't materialize you can still export him without losing money. I don't think you can expect much more, considering the demands upon top clubs these days.
I wasn't criticizing United fans in particular. It's the same everywhere. Even the fans of my clubs aren't willing to risk too much on our youngsters, they prefer to pay 5-10m for a player who is already a star in Argentina or Mexico and who will give us bigger guarantees. If they're indeed that good, the cycle continues with them being ready to step up for even bigger clubs.