You can even give the Steam key with a purchase on Itch.
A similarly frustrating thing is when a dev gives out ONLY Steam keys (no downloadable files) if I try to purchase here. A good example is the game Smile for Me. I loved the art style and was really excited to play it. Then I realized that purchasing it on Itch would only give me a Steam key…except they released it on Steam, Switch, Playstation, AND Xbox. Makes no sense.
But usually, developers are keen to release their game on as many platforms as possible.
Yes! Isn’t a sale a sale, no matter how many there are, or where they come from? Especially for indie games. And I’ve seen a bunch that release on Steam to low sales because their niche game’s target audience is on Itch, not Steam.