If you have a transition both at beginning and end, which should it move? Both? What if you did not want that? What if you only wanted one or the other? If you have a transition between A and B but not C and move B, should it move the transition? What if you wanted to keep transition on A and now make it transition to C? Should it only move the transition at the end of whatever you move? You see, there are many combinations, and some people will expect one thing and another expect a different thing. Still, others might want the transition to be simply automatically deleted. It makes user experience somewhat complicated.
Also, you do not need to remove a transition and recreate it to adjust its length, but the ability to adjust it may limited by the amount of unexposed additional footage before or after the transition.