Nobody wants to admit just how bad the bot problem is, because it starts to dig into the fact that advertising isn't nearly as effective as advertisers let on

Taking a blind guess here because I have never worked at Google, but I would assume there is one organization that has the data, and another organization that can block IPs from clicking on the ads and consuming the spend. There is a byzantine process preventing that second org from getting the data along with a lack of motivation because it would decrease ad-spend, which is one of their key metrics. Org2 which deals with people clicking the ads is a bad place to work and no one who is actually good sticks around long enough to navigate the process and implement this, so the can gets perpetually kicked down the road.

Tends to be how it goes once you reach a certain size.

Because doing this would reduce their profits.

Admitting in public how bad the bot and fraud problem would be, metaphorically speaking, shooting their primary revenue source in the dick.