Lol you don't "buy game" here, you "donate money" to support the author. The money part is optional, or at least, it is, in this game. The website itself stated that.
Secondly, I see you are referring to ALL games on Itch. As a matter of fact, a lot of games here do cost money to buy. You enjoy free games, you do you.
Thirdly, you do not understand one simple fact that these games take efforts to make. Sure, you'll get the game eventually, but that's not the point of joining Patreon. Patreon is simply a way to donate, support and express gratitude to the game maker. It's completely optional. If you don't want to pay, then enjoy the public version, that's it. No one forces you to.