You're making the same point that you're replying to. The juiciest users pay, so the non-paying users are the penny pinchers that convert way less on ads, so the ad revenue is obviously very low compared to the revenue from the paid users.
Similar to how people self selected into iOS and android and to this day its way more effective to advertise to less price sensitive iOS users than Android users with cheap phones, though the effect was even larger in the early days.
Similar to how the CTR of ads on msn.com or live.com was once way higher than everywhere else because anyone who was fooled in to using those default start pages was probably more easily fooled in all aspects.
Similar to how people self selected into iOS and android and to this day its way more effective to advertise to less price sensitive iOS users than Android users with cheap phones, though the effect was even larger in the early days.