If you've got all the modules for a build in local storage, ready to go, why wouldn't you just put them on a ship?
Honestly, it's not a terrible idea... in theory.
I mean, at the very least, you've got to wonder why ED doesn't have it's own, internal, version of coriolis.io - something that'd allow you to theorycraft ships and, ideally, could start to piece them together from stored modules and add modules sold at the local station.
It certainly could be done but in a game that's all about building and flying spaceships, and where players can easily obtain as many credits as they want, surely it's easier just to buy another ship and create two separate builds?
And, of course, FDev probably want us building multiple ships because that'll pad stat's for things like player numbers and time in-game.