Fortnite uses both EAC and BattleEye, so it really isn't that easy to integrate with their custom solution. Also, they have to test it to make sure no bugs are introduced. Afterall, it's a multi-billion USD game.
But as we know, they really don't care, so even if it was only a day of development time, they wouldn't do it.
Just as a heads up, while they did drop the Linux client, unless it's been changed very recently you can just use the Windows version through Proton/WINE and play that way.
Which isn't an excuse for what they've done, I only mention it in case you or someone else didn't already know.
I highly doubt they'll find any bugs even if they did test it. Even if there were bugs, most likely Valve + the community would be the one's patching it.
Epic doesn't want to try because they have a stick very deep up their ass.