Arguably they would be more materially advantaged if they were forced to KYC/validate ages, not the platform; because sure, there's a cost to doing it, but presumably having hard data on who your customer actually is, with age and address and everything, is worth a lot more than the verification cost. And being able to say "We're legally required to gather this" gives a lot of PR cover (even though it'd be followed with "but we're giddy to do so and we will abuse this data and you every way we possibly can. No one at Meta believes you are human. We hate you as much as you hate us, but we're stuck in this together, endlessly loathing the supernatural force that keeps us working together.")
But, On the flip side: I also don't doubt that Meta is doing this, because the purpose of a system is what it does, and the leadership at Meta has done nothing in the past four years to demonstrate that they're capable of cogent thought and execution. We want to believe there's some evil plan, and maybe there is, but in all likelihood one day we'll learn that they're just... unintelligent.