I've often wondered why I can't set my campaigns to, e.g. "Only show this ad to people who have bought something from some vaguely legit website in the last 2 y

I've often wondered why I can't set my campaigns to, e.g. "Only show this ad to people who have bought something from some vaguely legit website in the last 2 years" You can assume that any impressions you were going to get from the people this excluded are pretty low quality anyway. Ad Tech folks: Why doesn't this already exist?