And yet it happens, which is to the point and (depending on locality) people are still held accountable. Adopting nascent technologies almost always comes with risk tradeoffs because we don't yet have a solid understanding of the latent risks. Look at aircraft statistics if you need an example. To your point, over time they will improve, but not without accepting additional risk in the near-term. The last point is false only if you believe we understand the risk profile enough to unequivocally say autonomous cars are safer in all conditions. I don't think we can claim that yet.

What you're advocating is like someone in the early 1950s being told "Just make everyone fly in a de Havilland Comet! Don't you know air travel is going to be the safest mode of travel per mile!?" While true over the long run and in the aggregate, that individual on the Comet is being told to take an outsized risk. I'm making the claim that some people (not even necessarily me) would have a moral issue with being told at the individual level they need to take a larger risk for the betterment of society in the future.

So even while in the long time horizon they may be safer, or even be safer in the aggregate now, you could be subjecting passengers to higher risks in those individual cases. I can't speak for Kant, but I think some people may think it's immoral to draconianly force them to accept that risk. Kant speaks against the hypothetical imperative which is a motivation to obtain certain ends (like forcing policy that reduces traffic deaths in the aggregate).

If you disagree, ask yourself or others a hypothetical: take whomever you love the most in this world whether it's a child, a parent, or a dog. Would you be willing to adopt a rule today that they can never step foot in an ape-driven car for the rest of their life and every instance of travel be relegated to autonomous vehicles? Even if you are okay with that risk, there are plenty of people who would balk at it.