Money. They make so much money from malvertisement that it doesn't make sense to remediate them. It's corporate greed and they've known about this since the 2010s, various campaigns have sprung up and have caused untold X amount of dollars in damages in breaches, and nothing changes. I've heard at least one FBI agent who had investigated cases related to malvertisement put Google up there with some of the worst ad networks.

This is it. The truth is Google's largest revenue sources are scams, fraud, and malware, and they wouldn't be in business without them. They're not going to remove all of their highest pay per click customers because it might harm society or something.

Just try to convince Google to remove "MapsQuest Driving Directions", a browser hijacking extension advertiser who clearly impersonates one of Google's competitors: It's an extremely high value scam targeting seniors (the folks who mostly look for MapQuest on... Google) and it makes them way too much money for them to stop it.