If they do explain why, please don't say anything I don't want it spoiled for me.
They don't even mention it, but I'll take a stab at why: Alternate Universe setting, so not every little thing in "our" universe exists in the Fallout universe? As in, same thing that explains
thousands of other differences both large and small?
So would have you liked to have the by-pass in the game?
For what purpose? So we could have yet another blocked path across the river that we can't use? :shrug: Meh.