I am not an advertiser or anything, but businesses dont "trust" them. The moment businesses see an alternate option with more transparency and somewhat similar results - They will jump ship. Although we are way far from anything like that coming up (on that scale)

There were a bunch of ad marketplaces that were pretty transparent - (buysellads.com? not sure, and many others) because they were pretty basic, you were buying ads directly on websites. Google is not transparent on the publisher or the advertiser side. Advertisers "bid" on "keywords" with the amount that google suggests (pulled out of their ass? who knows). On the publisher side, if you add "auto ads" on any website, a bunch of ads appear all over the place, most of the time breaking your page layout. You are given an RPM for your ads that , as far as i know, could also be pulled out of their ass. They control the inflow and the outflow, and adjust according to their cashflow needs. Advertisers like them because they provide a lot of bloated dashboards with fancy graphs as proof of their work.

I did always find auto-pricing for ads weird. Why would I trust the one entity that stands to gain from an increase to set my bid? The fact that it's impossible to verify the veracity of the system makes that a no-go for me, but clearly not for everyone.

Its not trust so much as monopoly access to the digital ad market. Google has so much prime inventory (search results + youtube) that it's impossible to avoid it if you want any kind of scale for your campaigns.