They do have those teams. I've seen them work; they've got power washers, and they blast the stations. They cleaned my previous-local station once every few months, at least.

The stations don't stay clean because they move millions of people per day, not all of whom are clean. Today I saw a kid throwing up on her way into the station; she and her mother were somehow polite enough to pause above a drain to do so. Combine that with people tracking upstairs dirt in, and littering, intentional and accidental (I dropped a candy wrapper while getting my kid home; I'm not stopping to pick it up mid-tantrum), and you've got a pretty messy situation.