Because that’s expensive and can be done with a camera.
Expensive, as in probably less than $600? Compared to the $35000 cost of a tesla?
(comparing the cost of the iPhone 12 (without lidar) and iPhone 12 pro (with lidar), we can guess that the sensor probably costs less than $200, so 3 of them (for left, right, and front) would cost probably less than $600)
lidar can actually be very cheap and small. Unfortunately, Apple bought the only company that seems to make sensors like that (besides some other super high end models)
There have been a lot of promising research papers on the technology lately though, so I expect more, higher resolution and cheaper lidar sensors to be available relatively soon (next couple years probably).