If United hadn't been run so poorly by the Glazers and were, say, 1 big signing away from being a genuine title contender, and that player needed to be a striker, then I think yeah you're right, go and pay £120m for Osimhen or the like. Liverpool did that when they needed VVD and Allison, and it was the right thing to do.
As it is, our squad is a mishmash of players who are all suited to different styles of play. We're overburdened by expensive flops in some positions, and completely lacking depth of any kind in others. We need to fill too many gaps to make just one big marquee signing, and PSR is a thing now, so we're having to try and be more creative with who we sign