You built a free-to-play game. 95% of your players will never buy anything. So you filled that gap with ads. Banner ads that nobody clicks. Interstitials that make players rage-quit. Rewarded videos that pay you $0.01 per view.
There is a third door. Your non-paying players are willing to trade three minutes of their time for in-game currency they actually want. Brands will pay significantly more for their verified, motivated answers than any ad network will pay you for their eyeballs.