They do though. The argument is two-pronged: (a) developers earn secondary fees on open markets, and (b) players get to resell stuff they don't want. It's literally a win-win.
Tim Sweeney knows.
If every games company is allowing users to resell their items, and Epic is not, they just look evil. We're not there yet but when we are you don't want to get the reputation of being the evil company that seeks profits above all else.
And competition doesn't/won't solve either problem, because third-party apps aren't why most people choose social media platforms, and reselling hats isn't why most people choose a game.
In other words, the proposed feature is something developers want, not users.
The perverse incentive of the "token" and making a boatful of money suddenly gets people concerned about developer's income.