If it’s so often better to have your weight farther back, why aren’t mountain bike seats located farther back?
Off-road biking requires continually shifting your weight. On descents, yes, you want your weight well back. Otherwise your center of gravity can too easily end up in front of the front wheel — and that’s when you’ll sail over the handlebars.
But at other times, you need your weight more forward. You can’t steer if the front wheel isn’t getting sufficient traction. On ascents, you need your weight more forward or you’ll do a wheelie. The standard seat position works well most of the time, and puts you in a good position to shift your weight when you need to.