Pumps are not good at stopping and starting. They like to start, run for at least a few minutes, and then stop. Each stop/start cycle adds wear to the pump, as

Pumps are not good at stopping and starting. They like to start, run for at least a few minutes, and then stop. Each stop/start cycle adds wear to the pump, as the components inside it are at peak strain and friction when they speed up and slow down. Forcing a pump cycle each time someone opens a faucet would cause a lot of maintenance. This is why wells have pressure tanks, to allow the well pump to run longer cycles, building up pressure to be released over time in the tank. There's a second issue in that pipes don't like to be empty: exposing hot and humid surfaces to free air is a bad combination.