You actually could. It would take a very strong, very heavy airframe that could handle any stresses generated by the intended selection of engines. It would just be overbuilt and get terribly fuel hungry for most engines, while being optimum for one. This isn't financially acceptable for an industry where operating costs are razor thin (all driven by the fact that a $10 difference in airfare can mean losing a passenger).
Aircraft safety margins are 1.5 (built to handle 1.5x the maximum expected load), while bridges are 5+. The latter aren't constrained by weight, so you might as well go to town with a cheap, heavy design. A lower safety margin is only acceptable though when the expected loading is extremely well defined with very low uncertainty. This takes extensive testing (years for an aircraft), basically reducing the sigma by increasing the sample size.
Aircraft safety margins are 1.5 (built to handle 1.5x the maximum expected load), while bridges are 5+. The latter aren't constrained by weight, so you might as well go to town with a cheap, heavy design. A lower safety margin is only acceptable though when the expected loading is extremely well defined with very low uncertainty. This takes extensive testing (years for an aircraft), basically reducing the sigma by increasing the sample size.