My understanding is, it's a multi-party cat-and-mouse game that never stops evolving.
The publishers (websites or apps owners) that provide free content to users need a way to support the free content, and typically it's either donation (like wikimedia) or ads. To most publishers, ads revenue is far more predictable. By right most publishers also would like the ads to be reliable and relevant to the users consuming their content, but that's usually a secondary concern, compared to the revenue.
The users viewing the content are often annoyed by the ads, but as long as the ads are not too intrusive, they usually tolerate the ads. If the users occasionally find ads are actually useful, that's great, but most of the time in my experience, when I click into an ad it's usually a mis-click, not because I'm genuinely interested in this ad.
The advertisers need a way to reach more potential customers. Most advertisers are honest players in the game with real products or services to offer to users, trying to find ways to maximize the impression, click-through and conversion rate by optimizing their creatives, targeting, and bidding strategies etc. But there are always scammers that try to cheat the game, showing dodgy ads to rip off users whenever possible.
Google as an ad-tech platform sits between these 3 parties, trying to show the most relevant ad from advertisers in a publisher's ad-slot to the user. But the system has evolved into a complex maze over the years that likely nobody knows the full picture any more. New legal requirements and regulatory policies and industry landscape shifts keep pushing the system to extremes. There are teams to fight ad spams, but there are also business goals to make more money to fund the AI initiatives. The scammers keep finding new loopholes in the system, and the teams fighting the scammers are often steps behind, due to limited resource and budget. End result is many dodgy ads keep slipping through the policy filters.
In an ideal world, ad-tech platforms should optimize for satisfying all 3 parties at the same time: publishers, advertisers, and end users, in addition to making money for themselves. But that's probably hard to achieve, given the dynamic nature of the world. AI can help fight scams, but AI can also help finding loopholes in the scam-fighting system. It's a sad state of the world.