Sharr Khan said:
For the same reason you don't reinvent the wheel. If it works chances are we'll continue to use the method that has proven workable and or efficient right now.
Why would people in the 23rd century want to copy methods of production that are hundreds of years out of date?
Actually things don't change that much, its a mistaken conceit of scifidom in general and Trekdom in specific that thinks: "Future equals totally complete way of building or doing things".
I'd suspect this would increase with things like anti-gravity generators and such. Not having to worry about oxygen and being able to get at the guts of the saucer or nacelles would be served by ground construction then assembling all the pieces in space later.
It's true that methods aren't completely discarded. But how many methods and materials used to build castles in the middle ages are used to build modern sky scrapers? Not many.
The same would be true for building space craft. Espcially if one extrapolates from other things about space craft in today's world and the space craft of TOS era. For instance the means of propulsion is completely different. Why then would one expect the means of construction not also to be completely different?