Because the LLM based appsec products I've tested on a large code base have been very good at finding what devs and appsec code reviews didn't.
Which ones, out of curiosity?
Humans find security vulnerabilities all the time too. The assumption is a lot of the misses are because highly trained security experts don't have time to carefully read and understand all code without fatigue.
Humans can't be forced to evaluate every piece of code 100000 times in an hour. LLMs can.
With infinite hardware, sure. LLMs are pretty expensive to run vs fuzzing.