Who said that every part of the NX-01 was built in space? All we know is that, like the 1701(r), we see the final touches being applied in space. I don't recall anyplace in that series that stated that the nacelles were built from scratch in orbit, or that the main bulk of the saucer may not have been built planetside and lifted into orbit. We only know, really, that when Archer and Trip were flying around the ship before launch day, when it was nearly completed, it was in space. That's not really evidence of anything.