A quick search (haha) shows that google pulls in $182/year per user and 90% of that is from advertising. So yes google makes a lot more than people think from ads. I also suspect that is heavily lopsided, with certain demographic being more sought after by advertisers. If they were to offer a flat opt out rate, would need to account for those people switching away and losing a very lucrative market. I would even imagine kids are unfortunately a target for advertisers because they can be very impressionable, and become lifelong customers if convinced early on.

In addition, a flat fee could lead to a slow on googles growth, as once a users pays, they can not find new ways to monetize that user.

Don't get me wrong, I love the option to pay a fee to opt out of certain ads. It really helps align the incentives of the user with the company, but unfortunately it would have to be expensive to average out the more lucrative ads.