I think you're conflating "can't" and "won't". They have not been focused on driving revenue up; if this were a company goal they could have done a lot more sales. They acquire sales of advertisements in different ways, and if they really wanted more money, they could have pumped cash into expanding their sales force to do a lot of heavy lifting. However, their eye is on making that sort of sales unnecessary through producing an ads platform that people can go to directly instead.