They are easily verifiable and hill-climbable.

Because pre-LLMs humans partially "autogenerated" kernels through hyperparameter search and in some sense eating the code complexity in return for performance, and thus built tools for the same automatic verifiability that is useful for LLMs.

In some other tasks, we never built the same level of automatic verifiability since the level of automation in creation being much lower meant it's not giving you as much of a marginal benefit. We prefer code readability and simplicity and such in say, web services, because, say, the database IO time is going to dominate. Here getting an LLM to write a cromulent C# web service is more difficult since it's not easy to automatically verify whether code is cromulent or not. So if you put up LLMs to it, you end up with slop (which works).

OTOH, in kernel design, you give it access to every perf counter, every observable possible and have it optimise all of them. And all are verifiable/hill-climbable and you generally don't give a crap if the code is readable or reusable.