Why?
The source is nothing more than a nicety, not a necessity if the engine in question is already relatively stable. Sure it helps middleware providers do clean integrations, but apart from that ummm? The one thing that makes source worth it is you don’t have to rely on Epic to fix issues if you are prior to release and need something doing ASAP.
But with the speed Epic gets through things, you could debate how useful it is even then. With Unity, yes source code would be very nice but would you want to deal with that multi-platform monstrosity that requires 500 staff to keep going? You’d need a lot of man power to leverage it most of the time.