It's not like climbing on a road bike is all that difficult. You have minimal weight and never see a very steep hill. They just don't lay pavement over anything terribly steep except in rare cases. Roadies consider a ride "hilly" if it gains 100 feet per mile; hiking trails here are "average" when they gain 1,000 feet per mile. Mountain bikers aren't limited to the tame pavement, and most of them don't tell themselves they need $2,000 wheels to make it to the top. The last glacier I climbed started at a 30 degree angle and kicked up to about 55 degrees.