A camera is a dense, cheap, passive sensor. It reads signs, lane markings and traffic lights — information encoded visually and available no other way — but must infer distance rather than measure it, and it fails in glare, darkness and heavy weather.
Radar measures range and relative velocity directly using the Doppler shift, penetrates rain and fog, and is inexpensive and mature. Its angular resolution is poor, so it knows something is ahead more precisely than it knows what. Higher-resolution imaging radar narrows that gap.
Lidar builds a precise three-dimensional point cloud by timing laser pulses. It measures geometry directly at high accuracy and works in darkness. It is affected by fog, rain and spray, and it has historically been the expensive sensor — the cost has fallen substantially with solid-state designs.