It's because someone 40 years ago thought it was a good idea, and nobody really thought it through. Today it exists primarily because it exists and nobody's been able to generate the consensus to change it. It was nearly eliminated a couple of years ago, but ALPA shot it down by raising fears that deletion of that requirement would lead to a rash of major air carrier gear-up landings. The folks in the cognizant FAA HQ office still want to change it, but they're waiting until the hubbub from the last attempt dies down and they can produce appropriate data to show that the introduction of that requirement made no difference in the air carrier gear-up accident rate.