Google gets paid to show the ads.

Adding friction to the process (on the advertiser side) means less money for Google: 1) due to the cost of implementing the checking, 2) because some legitimate advertisers will be falsely denied, 3) because the scam ads were still paying money.

Not adding the friction doesn't seem to come with any serious downside. One would expect that it would, e.g. due to publishers removing the most profitable properties from the ad network or governments legislating this out of existence, but that doesn't seem to happen.

So, given these incentives, the outcome is inevitable.

If Google had to pay a massive fine for every scam ad that gets through, or was liable for the damages the ads cause, etc. - or otherwise had a strong incentive - suddenly there would be enough resources to make sure every ad is reviewed, advertisers that push scam ads get fully banned on the first offense rather than being given second, third, and fourth chances, etc.