How much time have you spent volunteering on trail maintainance? Have you ever built or designed a trail? Professional trail builders I know would not agree with you.
The existing trail systems were not designed for the kind of abuse that mountainbikes can do. Bikers will find cuts, alternate switchbacks, and rut out that fun mud hole. properly designed and planned trails take this into consideration, and bikers still find ways to create problems. Working on trails I have seen bikers do more damage than horses on certain surfaces, slopes, ridges, and wallows.
For every one considerate and concientious mountainbiker out there, there are four of you.