It's not dumb! lol You have to have a calf to get milk. After a while, milk production slows down and the quality of the milk goes down too. It's best to breed yearly or so - I don't know how long they would go without being bred again, but I think they could go a year, maybe two (depending on the cow).
We're getting a cow this year, a Jersey cow, and a person we know that lives about 30 minutes away from us has a bull he "lends" to people lol.