Well if you look at it that way, a microwave oven is say 1000 watts, to make it safe outside the microwave you need to drop that to 1watt, that's 1/1000th the power. Only 10db attenuation required. 10db is not bad from a communication standpoint as long as the available signal strength is modest, cell phone towers are everywhere and are capable of putting out decent amounts of power, and again modern receivers have come a long way.
Also think about the edges of the microwave door, they're basically long narrow slits, more than long enough to let relatively decent RF power through.