???
Ask any of the astonaughts or cosmonaughts working on space station Alpha how easy it is to build ANYTHING in microgravity... Besides, you still have to blast the material used to build the things off of SOME planetary or planetoid's surface, or tow asteroids in from somewhere (which still costs a great deal of energy). Simply saying that because it's a microgravity environment doesn't mean that building such mammoth structures is easy, or even feasable.
There's a host of troubles involved in such macroscopic engineering projects such as uneven thermal expansion, maintaining engine alignment during superstructural flexing, or even the added effects of such flexing.
And to say that these vehicles don't ever deal with gravity is a falsehood. Even their own "artificial gravity plane" will still exert the same 1G acceleration on the ship's structure the same way as everything else within the hull (Quote from Kip Thorne: "Any particle or wave will either interact with all other particles or waves or none of them").
My final point is that gravity is nothing more than acceleration, what about when the ship's own engines fire and accelerate it along it's direction of travel. It is going to have to deal with the exact same stresses (or possibly even greater stress if the acceleration exceeds 1G) as if it were on a planet's surface.
