Try building a 17' long smartphone some time, and see what problems you run into.
Manufacturing tolerances are a real thing. A tricky thing at that. Steel and aluminum expand and contract based on temperature. Big parts expand and contract more than small ones.When you bend sheet metal, it doesn't precisely follow the die, and it also springs back when the pressure comes off. Not always the by same amount, either - since the thickness varies, metal grain size varies, etc. When you have multiple bends in a single piece, the variances start to stack up. When you weld one piece to another, they change size on you while you're laying down the weld. When you have multiple pieces welded together, those variances also start to stack up.
The tighter you design your tolerances, the more precise the tolerances on the equipment making the part need to be, and the more expensive and slower things get. It's a trade off - cost vs. precision vs. speed.