You mean it wasn't profitable for them.

I don't know how seriously you were asking this question, but this probably works specifically on roaches because their legs are somewhat autonomous, but still controlled by the brain. So really I would guess it's the sort of situation where if the brain is not sending the legs commands, they can be manually directed (e.g. like being towed by a wasp). Now whether it would work on Humans is another story. While walking is mostly a passive brain activity, the brain is still required for walking to happen. If a Human were dosed would this, I would imagine something bad would happen aside from inability to walk, probably collapsing to the floor and the lungs ceasing to function. However, TFA did mention it only inhibited "complex" actions, so maybe breathing isn't considered "complex" (NFC to be honest).

However, if you wanted to make someone a zombie (at least temporarily), you wouldn't need to get them stung by a wasp in the cerebral cortex! There's a popular drug used for both date rape and robberies in South America known as "Devil's Breath"; usually some mixture containing Scopolamine. It basically puts you into a waking sleep where you are extremely complacent and easily influenced into doing things, with no memory of anything you did. Of course, these effects are only obtained in extremely high doses. So, there's also a pretty good chance it will just kill you instead.

When you see a wasp sting someone with this, then get's them to rob a bank, we're all fucked.