Light heartedly off topic, but at a decelerators conference last year I had a chance to talk to one of the JPL Entry Descent and Landing peeps, and he mentioned

Light heartedly off topic, but at a decelerators conference last year I had a chance to talk to one of the JPL Entry Descent and Landing peeps, and he mentioned that in Q&As, no single audience utterance carried more foreboding than 'Why don't you just...' To answer your question more helpfully, having closed loop machine vision stuff is quite computationally intensive, though there is already some on the rover (to help negotiating obstacles or stop if it sees something like a trench (MERs would famously stop at the same time every day, thinking that their now-infront shadow was a pit)) but it's power intensive and expensive (both mass and cost of a dedicated odometry camera, for example) and the lighting conditions are very variable. The MERs developed a technique whereby they would 'shimmy' their steering every so many meters, and then with the rear facing camera snap a photo of their tracks (with shimmies) and infer distance travelled and slippage and so on. This stuff is often easier with humans in the loop as the terrain still throws up unknown unknowns when driving. Here's a pic of the tracks: http://photojournal.jpl.nasa.gov/jpeg/PIA14129.jpg http://photojournal.jpl.nasa.gov/jpeg/PIA14129.jpg I guess another point is that while it might be a perfectly sensible suggestion technically, there's only so much 'new' they want to risk per mission (the Entry Descent and Landing phase contained a whole lot of 'new' as we know) and they maybe just wanted to carry on doing it in a way they know works ok from before.