Your most successful customer is the one that no longer needs your platform, so a monthly fee or relying on ads does not work. You end up with a perverse incentive to make sure users only ever get "okay" matches, and never GOOD or even GREAT matches.
But if you charge a one-time fee, then the barrier of entry is too high and you won't get many people buying, especially once the people who are frankly undateable start bad-mouthing your app.
You could try a method where you only pay once you decide you landed a good match, but that's going to be impossible to enforce without greatly giving up privacy.
Tinder isn't making money on people who match with people successfully and never come back to the app. The managers tweaking the algorithm know this.