I think a simpler answer to the others given is because we can, and don't think it'll make the cars less safe.

We add ultrasound range sensors for parking because it helps, even though it's possible to park despite not being bats. You also could still drive with reduced vision, but it'd be better if you didn't.

Would we drive better if we had accurate distance sensors?

So we theoretically may not need to, but they help with the current problems we have.