That’s just reducing the ‘reward’ for doing the work more and more: now it’s not something that can be used for elevators, bridges, parking garages and player-made underground parking, but something that works solely for under-bridge boating.
And once again, you’re reducing the amount of players that would be exposed to it, so you’re getting less bug reports, so it takes longer to go from the boat-only implementation to something that can be applied on more places in the game.
Also, what happens when you dig holes from the bridge? What happens when you build a little dock inside and start wailing up with a sledgehammer, then try to build stairs? What happens when explosives go off under the bridge and the roof colapses? If it no longer aligns with the road upstairs, these are all going to be issues.