Vintage watches are different from modern watches - they've aged. Some parts can't be imitated - the dial for instance.
I've just worked on a 'superfranken' 5513 with a 1570 movement, Phong case and a Ivansupremedarkoverlordmasterofdeathandallhebehold's guilt 5513 dial - the dial probably cost well over £1000, but it still looked like an aftermarket dial. That's disappointing, considering the costs.
What can't be imitated in a vintage watch - not even by MQ, Phong, Darkivanmasteroverlordblahblahetc, or even the demi-god that is TheSociety:
The dial.
The insert & it's pearl.
The crown.
And that's it - A case can be made to look 95% gen- aftermarket hands can be 95% of the gen and most people wouldn't notice these things anyway. Clasps are different, but they are much less visible. Links and end links, even experts would not know the difference sometimes. A good aftermarket crystal will look almost the same as a gen - but again, most people wouldn't notice the difference even when you explain it to them.
Movements, that's personal choice - but vintage movements are expensive to maintain (said superfranken 5513 is not complete because the watchmaker who 'serviced' the movement did so without dismantling and cleaning it . . . ) and aren't necessarily very accurate or reliable.
Like most people, you may have a limited budget, and you are probably attracted to vintage watches because of the way they look.
Some people will build a franken from almost all gen parts - as yodog is doing with his GMT, but that's a big budget build, and it's a one-off - he probably won't do it again, at least for a while.
Because most people can't tell the difference between rep dials and gens, rep inserts and gens, or rep crystals and gens, they like to have something tangible that confirms and reinforces the high value aspect of the build - a gen movement fulfills this role as nobody makes a rep of the 15** movements.
The Ferrari Testarossa is a great looking vintage sports car. If you had to choose between an almost visually exact copy of a Ferrari Testarossa, but which had only had 85 % of the engine performance of the original, or a kit car which only looked reasonably like a Ferrari Testarossa, but with the original engine, with 100 % of that engine performance, which would you choose?