!) You can ADD a lot of non-Steam games to your Steam account. You don't need a Steam key for this. Look in the sub menu in the left below corner of the Steam screen where you can add a game.
2) Developers on itch.io will not just give a Steam key. Steam costs them money too - To publish the game, to sell the game, even to give it away.
itch.io and Steam are two different digital shops.
Like in the real world, if you buy something in a supermarket, you cannot expect another supermarket to give you the product for free too, or to have the shop buy the same product in another shop and then give it to you.