I've always been under the impression that the function of a crew was for maintenance and not critically essential to the operation of the vessel during combat operations. Given this, one could presume the level of automation on a capsule fitted vessel has ways of adapting to minor system malfunctions. A mission runner running missions all day has, in my opinion, no reason to carry a fully fitted crew 24/7 and probably leaves most of them at home. Running war patrols or work in one of the militias? Those are probably full of people.
One reason that I feel that robots don't make much sense as crew is that if a robot could do those tasks already they would have incorporated it into the automation design of the vessel, which implies that said maintenance requires more than just a good ol' scrub from a robot or a routine systems diagnostics check. I also feel that cost DOES factor into this, despite the fact that we do not have a very clear or precise understanding of the exact nature of how deep the level of automation runs through New Eden society. Also, if someone told you that your Thorax would cost an additional 20 million ISK to purchase with some kind of automated crew, I don't care how rich you are the idea of flying with crew suddenly seems a lot more feasible.