Why is the water flow more rapid out of a faucet on the first floor of a building than in an apartment on a higher floor?

Unless there is a water tank or booster pump you are going to loose about 0.4 psi per foot vertical. Given that most floors are about 10ft floor-to-floor that is 4 psi loss.

So, if a first floor unit has 30 psi water pressure, the fourth floor unit has about 12 psi water pressure if no compensation system is in place.