The point is that you could do that, with a roll of gorilla tape.
The point isn't it's more awkward to do and takes longer, the point is that the explosive force delivered to the bridge would be the same. (Actually slightly more with a gorilla tape covered explosive as it would marginally increase the forces on the bridge compared to just hanging ones.)
If you put an explosive inside the bridge, the force delivered to the structure is several times more. Thus it would make sense to have "pits" to out explosives into, not just hooks to hang them off of.