"Happy" is a funny word though. Google is legally obligated to release kernel changes and legally forbidden from shipping kernels that incorporate other people's GPL-incompatible drivers.
It is definitely true that there are other companies who aren't happy to follow the law, from the Allwinners who just don't care to the VMwares who see the law differently to the Samsungs who seemingly honestly didn't realize what the law said. But given that Google is a large, relatively old company based in the US, you can't extrapolate very much from them being willing and even happy to follow the law in a straightforward fashion. It just means they know what the law is. It doesn't mean they like the law, or that they like the situation that caused the law to apply to them.
The explanation most consistent with the facts is that Linux being under GPLv2 allowed Google to get Android to market quickly with a robust, portable, well-designed kernel, and the things we like about free software (ability to hack on it and modify it, likelihood of gaining others' improvements) aligned with their business needs at the time, as a new entrant in a difficult market—but that Google, as a company, has no particular idealism regarding strong copyleft.
If they want to ship proprietary drivers from other vendors, they can't legally do that with a GPL kernel, and it's implausible to believe that they'll do so anyway and wait to get sued and try to play lawyer games. The simplest path forward is to keep making GPL reimplementations of those drivers for now... and to start making a non-GPL reimplementation of the kernel for the future.
If anything, the fact that they have been pushing vendors to open-source their drivers or rewriting proprietary drivers shows just how much they would have a business interest in a Linux-quality kernel that isn't under the GPL.
I suppose the OP was also talking about the changes done to improve linux for datacenter workflows, afaik there is no obligation to release the changes there.
Newer versions of Linux get steadily better at performance, so it seems likely to me that Google is primarily incorporating their changes upstream to save effort over maintaining a local fork (and not risk that fork getting out-of-date), and not so much because they find that open-sourcing their code is inherently worth doing.