itch.io

That rationale is absolutely braindead. 
There are two problems I see with that. 

1) Premise is often extremely important in getting people to take an interest if your work. There's a number of authors who had one great premise and none of the rest of their works really compare - even later ones. J.K. Rowling is a good example of that.
If you've got a decent premise, you'd have to be a fool to give up on it. Who knows how good your next one will be?
2) You're going to get a reputation for unfinished works.

Besides, what do you mean "already did pay"? Patreon supporters donate on a monthly basis, or with every release. Sure, there are some people who subscribe, download all the works, and then unsubscribe, but you would still get their money again when you finish the product, if not every few releases. Devs are not incentivised to stop working on the game halfway through.

Besides, PhillyGames already have a finished game under their belt. If that isn't proof of their commitment to their products, I don't know what is.

The main reason there are so many unfinished games is probably the same reason there are so many unfinished fanfics. That shit is hard. It takes commitment. People lose interest in their work.
It's not because they're scamming people, they just don't have what it takes to stick with their project and see it through to the end.