For the same reason why you can run infinitely long in heavy armor, why you can't cut wooden staffs with a GLB etc etc ...
for the balance. ;)
Coz this is a game, and games are usually all about balance, not realism, kkthx bye. Why should there no penalty for the arbalest(that already is the best crossbow) ? To buff it even more comapred to the others ? ... No thanks.