Because of the principle of least privilege: https://wikipedia.org/wiki/Principle_of_least_privilege

All current age verification measures open up a torrent of attack vectors on user PII and privacy. Limiting the number of entities that are able to access data is one of the best ways to prevent it's leak or abuse. Don't let perfection be the enemy of good.

But therein lies the fundamental problem with surveillance capitalism. Until the sale of personal data/metadata is outlawed, the practice of targeting content based on an individuals personal data/metadata is outlawed, there is a highly punitive cost for violations and leaks that make storage outside core business functionality a major criminal and financial risk, and the compilation of this data by "intelligence" agencies it treated as a critical attack vector to national security – the attack on each citizens civil rights that it truly is – most privacy laws and regulations are just virtue signals designed specifically avoid the root causes, and further entrench the power of monopolies and incumbents.

FYI I don't believe Google sells user data. They sell products which leverage user data to give them a critical advantage over every competitor who does not have trackers in everyones pockets/computers, does not store their entire web search/browsing history, etc. It's in the interest of big tech to protect their market advantage (like ZKP, which would prevent competitors from having a new gov-mandated vector to compile user data).