When you’re google size and investing engineering and cryptography resources into building security features, applied at scale to a billion* users, you probably want assurance that this list will he maintained for as long as you want it to be maintained, AND for no third parties like HIBP to enumerate over all google emails.

* random number.

Google does not have a very good record of maintaining their non-core projects.

Publicly, yeah. But you really think google would depreciate some core internal service actively used for account security, as well as internal employee security?