We got big news in the self-driving car industry today when John Krafcik, CEO of Alphabet’s autonomous vehicle unit Waymo, said he was stepping down. Krafcik says he is looking forward to new adventures after a “refresh” period. If he wants to stay in the industry, Krafcik would be an ideal candidate to hook up with one of the numerous self-driving vehicle startups now vying to go public, including, as we scooped today, Argo AI.

Krafcik may well steer clear of that, however. The very factor that may have influenced his departure from Waymo—the question marks that hang over the technology—is what makes self-driving car startups unsuitable for the public markets. As my colleague Amir Efrati has chronicled here and here, the early optimism about self-driving cars being ready for prime time by the early 2020s has faded. Training the cars to react appropriately to obstacles they encounter on the road turns out to be pretty difficult.