Because it’s great fun for rider and dog! Truth is, like all things really, there’s no one single truth (other than this one obvs). At one end of the scale you have well trained, properly bike socialised dogs (like mine), at t’other end you have out of control, slobbery and/or bitey hoodlums (& their dogs..) The former are fine on trails. I regularly take my lab/Shepherd cross with me, never had an issue (& yes on the rare occasions that she relieves herself during the ride I clean it up). Equally seen riders with dogs that they’re struggling to control. Worth pointing out that if you ride in s9mewhat dodgy areas from time to time, it’s no bad thing to have a fiercely loyal dog by your side..