They have an interest in showing you non-shitty ads. They sell non-shitty ads. Therefore they have an interest in showing you their ads. They also own the most

They have an interest in showing you non-shitty ads. They sell non-shitty ads. Therefore they have an interest in showing you their ads. They also own the most popular way those ads are shown: through their web browser. This browser has the ability (at Google's sole discretion) to block shitty ads, of which all of them are run by their direct competitors. So what's happening is, Google is giving themselves the right to block parts of the Internet they don't like, with the implied statement of "if you want your ads to be seen by Chrome users, buy them from us or else we might block them". That is a classic protection racket. "Want your ads to be seen? Better buy them from us, or... bad things will happen."