Assuming you're not trolling, there's a few other things to consider -
1/ User targeting is complex - you can charge more for ads if the users you're showing the ads to click
2/ Ads impact user retention - you need to balance making money and keeping users around
3/ AI generated ads - this is a pretty big thing now, where instead of bringing your own media, you just describe your target audience and the AI will A/B test media + CTAs for you
4/ Integrity - you want to vet the ads against laws/site policies
Probably forgetting a few, but there's a reason the ad industry employs so many
I've heard of the ad auction and assumed the point of the auction was to maximize price.
As for integrity ad platforms don't have any. Most of the ads seem to be for scams. The first search result for OBS is usually malware. Scammers have a low cost to advertise because they use stolen credit cards. Advertisers don't mind because the charge back doesn't cost them, COGS is near 0.