My point is that they already know how old you are, within some confidence interval, even if you never tell them or you lie to them, because they actively watch what you do and classify your behaviors with your age cohort. So why do they care so much that they gain another signal that only says "the user is over 18" rather than a much more valuable signal like "the user is 36 and lives in Albany" that they'd gain by doing the KYC internally?

I don't think absolution of legal liability has ever crossed any of these fools' empty heads. The threat of being fined & punished by the USG for doing something bad hasn't been a factor in corporate decision-making for decades.