I think plastic was used for two reasons:I am with you!! I bet you really hate the JK's and JL's those fenders are all plastic. I am not a fan.
1. Plastic is much more giving when it comes to rubbing them against rocks and trees. Theoretically they should pop back into their factory form. Since the JK is significantly wider than the YJ or TJ is was bound to have issues going down established Jeep trails without having the fenders experience some damage.
2. Plastic fenders/flares are vastly cheaper and less time consuming to replace when compared to a bare metal fender that would require prep, primer, paint, and clear coat.