So every grave opens, and they come climbing out? Maybe. But that explanation doesn't work with the flicks that base transmission on a bite or a virus (say, Shaun of the Dead, among many others). Nobody gets dug up and bitten in those movies.
They only come climbing out to the extent they are able to escape and have the ability to actually move. Zombies can die from starvation (I'm told), so most of the zombies that live in graves will never see the light of day.
The thing is, people *assume* a bite from a zombie transmits the disease, but that's not the way it works. See, the film "Fido," explains that the zombie disease came from the trail of a comet through with the earth passed. The comet trail infected the atmosphere of the planet. Everyone is a zombie now.