Y'all need to understand that he can't give a steam key for everyone that bought the game here
If Mike could do that, he would probably just ask "Hey, you can give the rest of the money so I can give you a key"
Getting a Steam key for your own game isn't as simple as "Oh I want x many keys for my game", that could cost him in terms of someone leaking the keys (which happens a lot) and people being able to get a pirated version and many other risks
BUT, even if he could do that without any risks, it is against Steam policies (not hard to guess, he would be avoiding the 30% Steam cut for each sale, yada yada yada)
So you can stop complaining and buy the Steam version (or don't and stick with the Itch.io version, up to you)
I myself bought the game here and of course I would like to get the steam key for free, but that's not how it works unfortunately
Think of it as a donation to give a boost for Mike to keep maintaining the game/making it better!