I think the answer is that Apple killed 3rd party cookies on their browser- which includes every single website view from an iPhone, and Mozilla is working on the same thing. So Google, even with full control of Chrome, the most popular browser, could read the writing on the wall that 3rd party cookie tracking was on its way out. The adtech industry has responded so far in ways that undermine security completely- mostly trying to get their cookies counted as first party (which also means that their scripts can view the first party's data, e.g. they can steal your persistent log-in!).

So Google has been trying to come up with something that Apple will let them get away with. Either give up on targeted advertising to iPhone/iPad users or come up with something better than third party cookies.