The cynical assumption would be that they're just sitting on the extremely vast hoards of money and greedy for more. The (slightly) less cynical assumption is that their interest in Search nowadays is as a piggy bank for projects they consider more important.
Worth noting though the latter has long been the going assumption internally at Google: Search was the cash cow that funded Google's expeditions in finding the Next Big Thing. This plan has been complicated by the appearance that Google seems to not be terribly good at the kind of product execution that would lead them to the Next Big Thing.
The technology that one would get in an open source situation isn't very usable outside of Google's ecosystem because Google builds software on top of Google's stack. Like, without the monitoring infrastructure they've built or the Borg scaling infrastructure, their software is actually kind of fragile because the ethos is " If it starts to malfunction break it quickly so it can cause a monitoring event and to get replaced by auto restart."
The Google way of doing things is actually not a great way to architect most software that isn't running on a giant data center structure.
It's probably worth mentioning that hypothetically, One could take the source code and port it to third party libraries and kubernetes. But I can't help but think that that would be about as much work as clean rooming it from scratch based on a feature description.