It's because of the keeper. Even John Terry is going to get beaten in the air sometimes, which is why you want the bloke who can catch the ball above everyone's head to come and sort it out as often as possible.
This isn't news either, or one man's crazy opinion, a lot of United fans have known for a long time he doesn't come for crosses enough and it costs them goals.
Look at the derby last season for a perfect example. 1st goal, corner comes in on the 6 yard box line, Otamendi beats lukaku for a header. Neuer, Oblak, Courtois all come off their line and claim that ball.
Second goal ball flies harmlessly over every City player on the line of the 6 yard box where Lukaku tries to volley clear. Every other top keeper catches that ball for fun, unchallenged, no need to jump. De Gea rooted to the line.
That happened at least 10 times last season, I used 10 as it's the number of set piece goals they conceded, but in truth some of those won't have been his fault, while there will be some crosses from open play he cocked up for as well.