If I want to sell art in a gallery, the gallery probably takes 50%; sure I could sell it online, but no one will see it, whereas the gallery brings customers to the art.
Epic wants profit without cost at Apple's expense, and if Apple if forced to accept this, then everyone else will suffer, except Epic.
You could make an argument that 30% is too high, but it's not a binary choice, 30% vs 0%. Epic thinks that it is.