For the same reason Americans often say "Wimbleton" rather than "Wimbledon" and "liddle" rather than "little." It's called intervocalic alveolar flapping.
IMO, "graviTy" would sound too formal. Perhaps right if the character were a science professor - but even then...
And indeed, the vast majority of us do say the word with a d, not a t. It's a very normal, conversational, informal pronunciation, and to the point, we all understand the word even so.
