last season Fraser got the second most assists behind hazard
He did, but he's not the quality Utd need.
You already have two inconsistent winger-forwards, and players in support that are sometimes capable, often limited - or learning their trade. Fraser doesn't markedly improve on that: yes he's industrious, yes he can deliver a cross, but realistically he's a player that could be a squad option for a side with any ambition of challenging for major honours. The benefit to Utd would be cost and potentially short-term performance, similar reasons he was linked to Arsenal previously this summer. Even then, Utd would still need to build on that with real quality and in multiple areas. I think monies would be better spent elsewhere. I would also contend his skillset is more suited to a side like Bournemouth, rather than Utd who (should be reasonably expected to) dominate a majority of teams they face in terms of possession and chances. Fraser is a counter-puncher rather than a consistent creator; Utd have the former in abundance, less the latter.
Although there are only two standout teams in the division, all expected rival teams - bar Chelsea - have elite-level talent in attacking areas. Utd's options are limited by comparison and focus should be on defining and acquiring players who can raise the bar, rather than the addition of another run of the mill player to a core of players who are just that.
*Appreciate you haven't made any claims of Fraser's quality in your post, but given the context and Utd's current situation my assumption is that we are not talking about Fraser coming to warm the bench.
