By keeping Waymo separate, shareholders can make their own choice as to how much they want to invest in search/advertising vs. self-driving cars. They're drastically different risk/reward profiles.

Obviously this isn't something a company usually does, but that's because either a division is small, or it has significant links to the rest of the company, or there's a clear joint strategy.

But Waymo is expected to be huge, and there's really essentially zero link to the ads/search/cloud/media that is Google's bread and butter.