I agree it makes no sense.
It makes "no sense" if you look at it from a naive position. It makes a lot of sense if you think about it from the perspective of the other stakeholders:
Player sought after: I should hold out for the best deal since I have other options.
Player's agent: You should hold out since we have these other offers and we'll be ready for any club on deadline day if it comes down to this. Don't worry about starting anything until July - just enjoy your holidays.
MUFC backoffice: What's your actual cut-off Ole? "I want them training with us on 1st July." Well we'll try for that, but we don't have much control since the close of the transfer window isn't until later and the agents know that. Managers always pick the most competitive targets, why can't we just promote from the academy and save me the run around every summer. Yeah, we'll be on holidays and have to finish some things for the tour. [not a full accounting of everything going on lol]
I can't see how a transfer takes so long for one player to be signed, negotiations don't have to take weeks on end.
They do when you don't have any leverage. These players have other options since we are not league winners and have an air of incompetence about us in recent years. People in large companies deal with these "influencing without authority" situations all the time - you just never control the timeline.
And I can't see how multiple players can't be worked on simultaneously.
There is
zero evidence that multiple players are not being worked on at any point in the year, not just the closed season. These deals don't just appear out of thin air. We are rightfully not entitled to the inside of those negotiations.
There are negotiations going on all around each of us every day that we are not privy to. Many of these amount to nothing. I've had multiple informal talks with other managers at work over my career about switching to their team and we've decided not do anything. No one lets on the conversation is happening and it's forgotten about afterwards. This is happening over and over again at Manchester United's management and agents talk with established players and even the parents of young prospects.
The closest I've come to knowing anyone making even a step towards joining the club was when an acquaintance's child was scouted. Obviously the club want you to be able to share your pride and announce Wee Johnny got a trial, but the discussions leading up to this were kept hush-hush. If Wee Johnny's dad happened to be a scouse fan who was happy to sabotage his kids future (a Liverpool supporting Dad seems like pretty good life sabotage!) and decline the trial, then the club don't have embarrassment or some stupid news story because there is no record of the conversation (plausible deniability.)
But the evidence (press rumours) is strongly hinting that this is what's occurring.
The press are there to make a good story and their rumours are not real evidence. They hear bits and pieces, but rarely do they get the full story, so they have to construct a narrative. People are so used to the narrative that they buy into it, and it's entertaining. I find summer transfer season kind of fun because I learn about new players since we get connected with everyone, there are humourous debates here etc. but I never consider it reality. Transfers are complex just like life is not a good read for many football fans who only want some headlines to banter about.