Google doesn't control all advertisers. Presumably this applies to sites with non-Google ads too.
Also, if these ads are rejected immediately (or nearly so) by Google, it will provide that much more feedback that malicious advertisers can use to "improve" their ads that much more quickly.
Those download buttons look always the same. Oh wait, they changed their color. After years of blue buttons they are now green. So Goolge can detect your face in a crowd on a photo, but can not detect a download button on small ad?
Also, if these ads are rejected immediately (or nearly so) by Google, it will provide that much more feedback that malicious advertisers can use to "improve" their ads that much more quickly.