Because it's by far the dominant strategy for distributed trust-ranking systems out there, with decades of research around it. Might as well look at the forest when realizing that it'd be nice if trees existed.

And I don't think anyone actually trusts any major actor to verify anything, so a fully centralized system is likely out. Otherwise people would be hype about WorldCoin, instead of recognizing it for the stupendously malicious grift that it is.