30% is the industry standard across the board, with the exception of Epic which takes 12%. However, Epic has already shown that it's ready to dump loads of money into store exclusivity deals and tons of free games, so I will argue it's for the sake of growing the number of users and developers using their platform.
But do they, or any other competitor or similar store, offer the same functionality as Steam? rtxn already mentioned some. And there's more. And then there's the fact that Valve is using all that money not only to stuff the pockets of alread rich people (not that Gabe isn't a multi-millionaire if not billionaire, idk), but actually puts it back into the industry: Their own store, Linux/Proton (you may not care, but Microsoft becoming a monopoly in PC gaming is no good), and hardware (with their Steam Deck handheld, and VR stuffs).
Steam might be the biggest player when it comes to storefronts, but it's because they've actually earned it. And they're not actively preventing other competitors from entering the scene (other than existing). In fact, they keep trying, and keep failing, and then going back to Steam.
I'm not opposed to more money going to developers, but let's not single out Steam, who (perhaps besides GOG? I am not familiar enough with it) is doing the most for users and develpers.