No one is saying they're cheap, but the billion dollars in the last decade, limited continental success, and the Glazers continuing to suck crazy amounts of money out of the club mean they can't go splashing 150-200M in a summer right now unless it's funded by sales.
They just had a net 200M+ spend last year, they're not going to do that again this year.
They may end up with 100M net if they can ditch a good amount of salary as well, but that's also theoretically got to bring in 3-4 positions.
Uhh, United's owners are the absolute textbook of actively handcuffing their spending. United is so far in front (or behind if you want to look at it that way) of the "net dollars owners have spent" category they're basically lapping the field multiple times.
That doesn't mean they're poor, it means if they had been run competently they'd have had *another* 500+M to spend over the last decade in addition to what they already spent because they're an absolute commercial behemoth.
Edit: Obviously realistically, a competent team wouldn't have spent all that money just on players but would have spent on upgrades at OT and the training venue, but the fact that OT is crumbling apart is yet another thing that is slowly starting to catch up to the team.