They don't have enough data to brute force the problem with machine learning. The dataset available will never be large enough to allow ML autonomous cars to solve every situation a human driver can. Another AI breakthrough is needed.

LLMs are so good because they have almost the entire digitized text output of humanity available for training. They may not always provide factually correct information, but it is dished out in near perfect English (or whichever language you prefer).

Personally i think if we want to recreate a human driver full AGI is needed. But, in the right conditions current autonomous cars will work well. They just need some external helpers, like being geo-fenced to a set of roads which have all the correct sign posts and markings.