[Melonotromo]

Fortnite uses Unreal Engine 5, the most advanced Game Engine in existens which sourcecode is freely available for a few years now.

Open sourcing old stuff is a lot of effort. You need to find all people involved or know the legal status of the copyright. You need to go through all the code in case you had some properitary stuff in there which you might have paid for. And you need to do all of this next to what you normally do without a direct benefit.