"Why do cats does not sugar?"
Because of a defect in their DNA. In mammals, the sweet taste receptors are composed of two proteins, and TiR2 TIR3, housed on the surface of taste buds. But in the cat, cheetah or tiger, the receiver lacks the TiR2. Without it, the receiver is incomplete and is not functional.
The cat, however, in its DNA the genetic sequence, that is to say the user manual for its cells synthesize this protein, but this sequence is defective.
If your cat loves ice cream, it's because of the milk.