Self driving has a well defined, static problem space, with inputs that don't change often (how often do they come out with a new street sign?).
Twitter is combatting distributed adversaries who are constantly adjusting their approach in evading detection.
One of the things that makes safety critical applications like self driving so hard is they have such an abundance of low probability, high severity cases that it is very difficult to define/test them all.
Oh, that'll come.
They problem is so ill defined that it sometimes has to be modeled as random events instead of a deterministic physical process.
Bots on the other hand changes constantly - what is the definition of an abusive bot? What is the definition of spam? The adversaries on Twitter adapt not only their tactics but also their goals.