When speaking of evolution, why do some people protest that it is "only a theory"?

The statements in a theory are already true. They are empirically based, made in a way consistent with the scientific method. All the word theory means is that although we know something about a phenomenon, we do not know all of it.
For example gravity.
It clearly exists, and we can accurately predict its behaviour mathematically-so accurately that we sent men to the moon and robots to other planets. We simply do not quite understand the exact physical nature of the attraction between bodies in the universe.
But that fact does not make gravity non existent. It simply offers opportunities for more research and understanding.
But when speaking of evolution, some people seem to forget the nature of theory. In that particular case, they dismiss evolution as “only a theory”.
I doubt if they would get away with saying “Gravity-Ha! It is only a theory!”
So, how do they get away with this obvious disregard for principles of epistemology, that have been used successfully since Aristotle?
Why is this particular show of provincialism given more latitude than if they disputed, say, gravity?