I mean kernels in the sense of functions.

> Why wouldn't GPU parallelization make inference faster?

Because most DL models are deep, and not exactly wide. I've explained already, deep means a long serial chain. Not parallelizable outside of data parallelism, which doesn't speed up inference, and model parallelism (generally not implemented, and has heavy IO costs).

Wide models and how they become equivalent to deep ones are unexplored, although they are theoretically just as expressive.