Earlier quoted context omitted.
It’s probably because advertisers demand that their ads are not placed on “bad” content. By limiting ads to 1000+ subscriber channels, it’s much more likely the content isn’t in violation of the YouTube rules.
All content is advertised on by YT, they just don't start sharing revenue with you until you have 1k subs.
Likely it was a decision based on analytics that Google, and no one else, has.
Imagine we are talking a cable TV platform instead, and your analytics show that most people flip through channels most of the time, as opposed to actually watching something specific. You make a rule to only share revenue with channels if the viewer is engaged longer than 120 seconds. People are still watching TV, you are still monetizing, but the channels aren't getting fully paid and only you know the money involved.
Making people feel they need to earn the right that their earnings be shared with them is just icing on the cake.