Because it's a very limited, mostly useless exercise, unless you've already developed a quality physique. You are not likely to get substantial results from it, and it's time you could spend doing more effective exercises.
I mentioned in another thread that I do not do dumbbell flyes, ever, and I wouldn't consider them unless I had 3 hours to spend in the gym every day, or I was a competitive bodybuilder. If you want to build your chest, dips are far more effective. If you think you have time/energy to do flyes after dips, just do more sets of dips instead.