Why do sprinkler systems keep working when it's raining?

This seems like a no-brainer, but all too often I'll be driving down the street when it's been raining all day long and I'll see someone's sprinkler system just doing it's job, keeping the lawn wet, when it's already wet from the rain.

Shouldn't there be a way to detect when it's raining? The brains of the sprinkler system are usually in the garage or close to the house, perhaps an internet hookup so it can query weather.com or send simple commands to the internet like ubiquity does. Seems like we could save some water that way.