The problem with that is scaling out the bridges will start to look weird with giant rocks or wooden planks in the bridge if we did it the same way that the trees can be expanded. Also the tree's expand uniformly, in all three axis at the same time, if we did that for bridges you'd see a giant square bridge.
I do have the ability to snap bridge pieces together added in TGC2 now though so laying out a new bridge will be much, much faster.