GCP, AWS, other IaaS providers, and more generally, any "post-paid" service company, needs your credit card, because the services these providers provide allow users to generate costs that can only be measured after-the-fact; and where those costs greatly exceed the free-plan limits, the provider wants to be able to pass those costs to the user.

As well, post-paid providers rely on credit-card billing information to deduplicate/KYC users. Without this sort of information, an, er, "ingenious" user could just sign up for a million free-tier accounts and lash them together into a quota-evading resource-sucking behemoth.

Sure. All that is true. And the credit card number ties your free GCE account to your profile with credit agencies that ad companies use to improve their targeting. It's not one thing or the other. It's both.

Do you think it's really that valuable for Google to spend engineering hours on an ad-targeting integration that will only improve targeting for maybe ~100k users at most?

And probably far less, actually. As many GCP users as there might be in the world, most of them are IT staff working for some-or-another enterprise; where that enterprise only has a single GCP billing-account administrator. Nobody else in the enterprise has their card on file. (And that billing-account administrator's card-on-file is just a corporate credit card, that tells you what the corporation buys, but tells you nothing about the individual. And their email is just a group/alias — billing@ or somesuch. Impossible to log into; impossible to browse the web as; no way to target ads at.)

You'd think the long tail of individual accounts could have more value, but those are the same users who GCP is least interested in recruiting to their platform, and the ones whose entered data is least trustworthy, because of all the spammers and crypto-miners attempting to use stolen credit cards to pay for service. You want to bind some poor random Joe's card-hubbed ad-profile to a spoke created by the person who stole their identity? That's negative average ad-targeting ROI!