Because when my bike has been locked up outside of my office for nine hours in a blizzard and the sun went down two hours ago, de-icing my derailers and shifters just is not very fun.

Because twenty minutes into a cyclocross race in which I am trying desperately not to finish last, the mental wherewithal to push the correct lever the correct distance in the correct direction to get myself into an appropriate gear is not something I possess.

Because the bushes that I invariably end up in because I am not very good at picking lines on technical descents like to grab hold of derailers and not let go.

My road bike has a bunch of gears, and I seem to be able to deal with them when riding along at comfortable speeds on paved surfaces on warm sunny days.