This is not my puzzle, and I don't remember the source, but I don't see it posted here yet, so...

A subway station has three escalators running side-by-side. One escalator only descends from the street to the subway platform, and the other two escalators only ascend from the subway platform to the street. Why aren't any of the escalators rigged to switch directions, based on time of day, train schedules, etc?

This is not my puzzle, and I don't remember the source, but I don't see it posted here yet, so...

Update: After reading some of the alternative answers, I realize this puzzle would be better with an elevated station, not a subway, so I have included both versions. Either version supports the accepted answer:

Original puzzle:

1. A subway station has three escalators running side-by-side. One escalator only descends from the street to the subway platform, and the other two escalators only ascend from the subway platform to the street.

Better puzzle: 2) An elevated train station has three escalators running side-by-side. One escalator only ascends from the street to the elevated platform, and the other two escalators only descend from the elevated platform to the street.

In either case: Why aren't any of the escalators rigged to switch directions, based on time of day, train schedules, etc?