depth is reliable

No it's not. World is filled with optical illusions that even our powerful brains can't process and yet you expect two web cams to do. And depth is not the only thing that's needed when it comes autonomous driving. Distance is an absolute factor. Case in point, two killed (if not more) bikers because they had 2 tail lights instead of one and Tesla thought it's a car far away instead of motorcycle close by. Ran them over as if they were not there. Us as humans would see this rider and realize it's a motorcyle... first because of sound second because our brain is better at reasoning. And we'd avoid the situation. This is why cars MUST have more sensors, because processing is lacking so much.