There simply are no advantages AT ALL for building a ship on the ground, so why would you?
How about not working in an environment that's nearly instantly fatal if you suit's seal or radiation shielding should fail?
A saucer, or even just a beam, is a whole lot more instantly deadly should whatever keeps it up and in place fail; either will crush you like a bug. In fact, neither a suit's seal or radiation shielding is anywhere near instantly fatal. There's more than enough time to get someone back inside, especially with transporters.
What about working in a gravity field that's natural for humans,
Which is what makes things MORE dangerous, not less, and is a detriment to building ships. The gravity field is a PROBLEM, an OBSTACLE which is why you would NOT want to build there.
not to mention ship-normal for calibration purposes?
Ship-normal is NOT inside of a planet's gravity field, ship-normal is being in SPACE. It is therefore much easier to calibrate a ship while it's being built in space.
There would be a lot of manual and skilled labor involved, probably requiring many specialists who aren't trained to use a space suit and work in free-fall.
No, everyone who would be working on the ship, would obviously be trained to work on it where it's built, namely in space. This is not a problem for us if and when we're going to build our first ships in a spacedock and/or at a space station. For a society for which space is old hat and has been around for two and a half centuries, this is something they wouldn't even be thinking of.
What about ease of transporting materials and personnel to and from the work site? Transporters probably use huge amounts of power, and that costs something, regardless of the economics of the future.
Which is exactly why you want to do it IN SPACE. Most of the materials by then, would be mined from asteroids, or on other planets than where the ship would be built. Why go all the way down to the planet where you're REQUIRED either shuttles or the transporters, if you can just keep it in space at the space dock never having to use them?
No, sir - I see nothing contradictory at all in building the major components on the ground, boosting them into drydock with antigrav or some such, and having the final assembly take place there.
That should no longer be a problem if you've read the above.
Of course, I'd like to point out, that the trailer did NOT show us them building parts and then moving them up (which would be bad enough), but they built THE ENTIRE SHIP on the planet. Which is utterly idiotic.
I can't grasp why people try to defend what the trailer showed us, when they themselves don't agree with and find it ridiculous, by rewriting in their heads what the trailer showed us. I mean, if you have to defend something by first starting "What I have to defend, I can't defend, so I imagine it was something else" you should pretty much know your point is indefensible.