Steroids give an athlete a competitive edge and can help them win a competition.
Photoshop is an editing tool used to make models look better in pictures.
Photoshop doesn't make them a better model in the photoshoot. When pictures are submitted to competitions, it's not just about the model but the photographer also. Photoshop is one of the photographers tools. If you said photoshop is illegal then what about camera effects? Would you police what sort of cameras are used?
Modelling isn't even a competition generally and when it is, it's usually on a stage where the focus is on the models in person and photoshop isn't a factor. If the competition was based on pictures then most of the work is done by the guy who took the photos and edited them.. that's part of the gig.
I guess you could make roids legal and then the bodybuilding equivalent of the photographer could be like a chemist or something?