Realistically, it's because for the last decade or more we haven't been as attractive a proposition as other big clubs so when multiple clubs are in for a player we often lose out.
From our perspective as fans, playing for United has an innate draw, but to a professional footballer with no affinity with the club? We aren't based in a fashionable area, we don't have top class facilities or a top class squad and the club is notorious for being run shambolically and for failing to get the most out of our best players. But despite that, and the fact that we aren't seriously in competition for the big trophies, we remain one of the biggest and most scrutinised clubs in world football. Players who join are constantly under a microscope and regularly lambasted by the media, our own fans and opposition fans for failure to live up to expectations which are, at this point, utterly unrealistic.
All that in mind , it's no surprise that we often struggle to attract the best players, especially when other big clubs are in the mix. We end up settling for second rate talents and/or paying over the odds to get players in who otherwise would have preferred a move elsewhere. The knock-on of that is that we have less leverage in contract negotiations with our existing players. We end up paying them more than they are worth to avoid them leaving and us having to spend even more to replace them, which means when we do want rid, we struggle to find teams willing to match the terms of their current contract.
And obviously, all the above is exacerbated by the inability of our scouting team to find talent that the entire world doesn't already know about.