It's not cool to ask in a store why people don't prefer to use another one. Still, I'll answer some basic things that you seem to not know and that make a lot of difference when choosing a store to sell games.
On Steam you pay 100 dollars and you can publish a game, you can use the Steam API and unless the player cracks the game, they can't play it without using the account where they bought it.
Steam has many more users than the other stores and therefore, it is usually the store where you will have the most sales.
On GOG, you can't publish whatever you want, you must send them your game and they evaluate if they are interested in selling your product or not. Your game must be DRM free.
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If you speak as a consumer, better buy on Itch.