To maintain sightlines you would massively sacrifice atmosphere- the back third of seats would be absolutley miles from the pitch. The cost also goes absolutely through the roof, exponentially with more seats. AFAIK BMD cost around £800m, but making it even 60k would've probably tipped over a billion. 100k capacity in a brand new modern stadium, and all it entails would be an absolutley eye watering sum, which is why Radcliffe is trying to get the gov to help fund it. In Spain RM and Barcelona have both renovated their grounds rather than totally rebuild, cost will have factored into that as well as all the safety kinds of stuff.
Generally clubs have realised its better to have a slightly smaller capacity that is always full, than a big one that has empty seats. Much like everything else at that club, Utd are behind the curve on that- or they are confident they could fill it every game.