Why? These clubs just totally have 2 different philosophies. The way they play is so much different and you guys will get so frustrated by Ziyech, because he wastes a lot of chances (however creates a lot aswell). That is why he'd fit with a team which will be playing around the opponents box a lot and/or who are gegenpressing.
A Van de Beek could fit United (but I think he'd fit nearly all teams), but he'll need 1 more year at Ajax to make the step conveniently. A Neres would fit aswell. He might look sometimes a bit dreamy (his face), but would actually be really good for United. He's fast, direct, has a dribble and produces a lot of assists and goals.
If you want a recent example ToD: Blind was not good enough for United, but with Ajax he thrives in the CL. It is because of the different playstyle.