I don’t know why Google have allowed their bootloaders this freedom, but I can’t imagine that a company with a reputation for killing anything they touch would allow it to continue for much longer.

They've historically always been very pro-developers, and they know Pixels are attractive to developers as sort of the defacto successor to the Nexus line, which was aimed specifically for developer as a way to have a close to AOSP ROM.

Google's also responsible for the entire bootloader specification, which does include provisions to provide your own keys and allow relocking the bootloader with a custom OS. So it's quite fair that their phones implements the full spec completely. If they didn't want people to be able to do this, they wouldn't have written a spec that calls for people to be able to unlock and relock. They also provide ways to authorize Gapps on any device/emulator, if they didn't want that they wouldn't let people do that either. But ultimately most people flashing their Pixels end up still using Google services and spending money on the Play Store and other Google products.

It probably also works to their advantage because it lets people poke at the security which helps people uncover bugs in AOSP so they can fix it.

The bigger concern is more about Google abandoning the Pixel project entirely rather than closing the bootloader.