Sure those roads are good for material but after said material are made into module blocks they would also need to be transported to somewhere else. Back when Fujian was still super blocks people thought they will transport them with a barge considering its direct vicinity of water. Most don't even think it's unfeasible or out right impossible to transport them by land and that's why later it caught everyone by surprise. Those blocks are already full width of the hull (exclude flight deck) and some of them are also pretty long. Just compare the road you marked and Fujian right next to it you could see those blocks would be wider than the entire road (this side was still empty land and swamp back then but other side of the basin don't have much difference either).
And the major confusing point is not whether these structures can be operate efficiently in their current location which they've already proven it's not a problem, but they take a position that could take advantage of water yet they choose not to.
Have you notice that they have launched several car transport ships from the assembly yard during the last few months? They moved the completed ships onto the large barge in the calm water of the basin, and the launched them into the water.
Advantages are: There is no need to worry about the tide and the fast current of the Yangtze river. Also there is no need for Shanghai Habour authority to impose river traffic control when ever there is a launching. There is no need to engarge MSA or CCG, and hire extra tugboat service due to strong water flow in the river etc.
They definitely have considered more thoroughly that you think. They think and decide collectively, there is a lot of thought in every decisions made. It is rarely a decision of an individual which could be error prompt.