Because not showing you ads would be "leaving money on the table" and that just won't do. If not showing ads wasn't part of the deal, ads will be shown. If not

Because not showing you ads would be "leaving money on the table" and that just won't do. If not showing ads wasn't part of the deal, ads will be shown. If not harvesting your personal data wasn't part of the deal, personal data will be harvested. That you paid money is irrelevant.