Does anyone know how vegetables came to be used to describe immobility/inactivity? I mean, I get the obvious sense, because vegetables don't tend to move on their own! But why vegetables in particular? I'm thinking of terms like couch potato, and the unflattering (to say the least) reference to someone "becoming a vegetable".

I think it's an extension of the medical term vegetative which means 'brain-damaged'. Ironic since in Latin vegetare 'to enliven', causative form of vegere 'to be alive' < PIE *weg- 'to be strong, lively'.

"couch potato", that's a funnny new to me one.
Potatoes are very 'lively'too.When you throw peels in the compost bin that still have the sprout beginnigs in them they will sprout and become potatoes.
So whoever lies on the couch too long risks etc......