Context matters.
How popular is the developer. What does the game's audience expect. Where will the game be released. And so on.
On Steam you can get away without a demo version, because Steam has no questions asked refunds and a entry bar ensuring a little bit of quality standard.
On Itch the preferred way of release is pay what you want. So a demo version is not even a thing. Also, those projects get published while in development. If you appear out of nowhere and release a game, you need a lot of marketing. It takes time to build an audience, if you are on a budget.