Because LIDAR specifically gives you the range or distance to each object. While in theory this should be possible with multiple cameras and stereoscopic vision/analysis, it obviously is not as simple in practice as it seems in theory. The additional depth info is also critical in identifying objects.
For example, several drivers of Tesla vehicles have been beheaded when a semi-truck turned/crossed in front of them and the car on autopilot evidently identified the white side of the trailer as sky and drove right under it, removing the roof and the occupant's heads. LIDAR would have identified a large flat object at range decreasing at the approximate speed of the vehicle, and presumably the self-driving system would have taken different action.
I think you misunderstood the question you responded to. The question was in response to (edit: your own conclusion) "ultimately, camera-only will be the best system" - and was wondering why the same camera system PLUS more sensors wouldn't be better?
edit: I think perhaps you didn't quite mean it that way, but it sounded like you were saying "eventually, camera-only will be superior to any other possible system, including camera + other sensors", which just seems nonsensical.
Ah, got it now, and it's an excellent point. Mostly thinking the idea of cameras(+intelligence)-only may be best in the end just from perspective of smallest effective resources and assuming biomimicry is best.
To your point, there are MANY situations where that will never be true, where the extra 3D info will absolutely make a difference.
Going back to the wheel/leg and fixed/flapping wing analogues; legs and flapping wings will likely always be superior for rough terrain and tight spaces. However, legged vehicles will never be as fast as wheels can go on smoother roads, and similarly, flapping wings will never be superior to fixed-wings+power for long haul or heavy air transport.
So, you're right — it's Horses For Courses — different solutions will work best in different situations.
For example, several drivers of Tesla vehicles have been beheaded when a semi-truck turned/crossed in front of them and the car on autopilot evidently identified the white side of the trailer as sky and drove right under it, removing the roof and the occupant's heads. LIDAR would have identified a large flat object at range decreasing at the approximate speed of the vehicle, and presumably the self-driving system would have taken different action.