Because recreational players would have difficulty playing with pro spec racquets.
Also a very small percentage of rec players know that the racquets sold are not the same as what the pros use. I've had many people playing with Pure Aero tell me they had Nadal's racquet.
I got a pro stock racquet before, and I found that it actually helps my game a lot. It gives me easy power and more depths on my shots. My retail racquet was much lighter compared to the pro stock racquet and gave me wrist problems, while the pro stock racquet is much more comfortable and easier to use! I felt like I can place the ball better and much more predictable.