Why do Christians refer to their god as Yahweh rather than I am when Yahweh is the hebrew word for I am?

Yahweh doesn't literally mean "I am." It comes from the root word meaning "to be," and means something along the lines of "I am who I am," although there are other possible ways to translate it. When using it as a name, it's just easier to call Him Yahweh rather than arguing over which translation is correct. Also, all names have meaning. My name means energy in Sanskrit. Doesn't mean people should start calling me Energy.