What you fail to understand is that these proven players won't want to go to Manchester United. For United to be able to pull these types of players, they need to make Saudi-level offers, and they don't have that luxury.
Currently, Manchester United are struggling to make it to the Champions League through the Europa League, top players don't want to put themselves in that situation. Look at a player like Bruno Fernandes, you can make an argument that he wasted his peak years in Manchester United. He could have gone to a better team like Madrid or Bayern and won trophies there. Other players are seeing this and wouldn't want that for themselves. The sooner people realise that Manchester United is no longer what it is, the better.

Also, I don't believe proven world-class players are what Manchester United needs. They need quality players, but they don't have to be world-class and proven. What Manchester United needs is to fix their club, their management, and work with a proven manager who can assemble a good squad.
The players at PSG are not all proven players, but they're heading to the semi-finals of the UCL. Look at the players Flick is using to dominate football in Barcelona, not all of them are proven players, but they're playing the best football currently. A good manager is an underrated role in football. The players in Barcelona were players that people thought were not good enough, but they're performing better than Madrid.