I didn’t know there was a Steam key gen floating around.
I would think the devs would want to get top dollar and just keep it on Steam, but I don’t know what the splits are.
It depends on the type of games as some benefit from being on steam for their services such as for multiplayer games and steam mods or in order to play together with others as some games can have separate player bases otherwise.
Plus when they do sell on other sites often times you still need to use steam to play it and you have to assume steam will limit those keys since they don't get any money that way.
Some sites like humble bundle are more for a donation type deal and the devs probably get less.
Plus if I recall steam enforces the rule that if you sell on other legit sites directly you have to sell for similar prices as you would list it on steam.
Some game devs abused the keys they got and gave them out for free to get a player base and positive reviews when the games themselves were usually horrible and steam cracked down on it.