>It's wild to me that advertisers are willing to use first party metrics.

I agree, and find it even wilder that first party metrics from Meta and Google are trusted by most major advertisers (including ad agencies). I'm talking about six-seven figure budgets spent without any third party validation.

I've seen some studies on click fraud[0], but when advertisers are effectively choosing from a duopoly that has limited incentives not to lie in their metrics, I find it strange that there are no popular, widespread and accessible independent validation tools.

0 – https://www.mdpi.com/2073-431X/10/12/164

There's a whole industry of independent validation tools - DoubleVerify, IAS, Human, etc.