It's because of the load involved. Flies you rarely see someone working above 160lbs total, whereas someone who can reach that top weight on flies is likely benching at the very least in the mid 300's. For a lot of people that aren't tricep dominant benchers, the overall stress placed on the chest by that larger load overcomes the increased tension you get from doing flies. At least, that's my interpretation of it. (by this argument, tricep dominant benchers likely would get a lot more growth out of fly motions than heavy presses)
It's somewhat like leg extensions vs squats. Most people, unless their glutes, hips, or hamstrings take the brunt of the load when they squat, are going to get a lot better quad growth off squats than leg extensions.