Money. Criminals and immoral people pay good money to show these ads. At least for Facebook, a court case regarding identity fraud has established that they knew quite well which ads were fishy, so they just charged those advertisers more. If I remember correctly, that amounted to about 10% of Facebook's revenue. It will be a similar case for Google, I assume. Sketchy ads pay too well to remove all of them and laws make it so the platform is only liable if they don't take it down after receiving notice of every single sketchy ad. Google, Facebook, and the others are all well near monopoly and built deep moats, so they don't really have to fear losing customers, because there's no simple alternative.