It's because a lot of devs have the talent to make these games but are making them as their side hustle. They weren't committed to finishing them in the first place and kind of just play along to see where their project takes them.
But that being said I feel like it's fair if the amount of time and energy they put into making a game show with the little donation they get. And if they aren't completing their games, people won't support or pledge. It's a catch 22 situation lol