Switching engines is a nightmare. Like… a NIGHTMARE. If you switched to the (infinitely more modern) unreal 5 - you’d be switching from an engine designed specifically to run wow to a generic engine. You’d lose all your tools - hundreds of bits of specific code to allow the game to work in a particular way, stuff to be imported, quests to be implemented, etc.
The time it would take to switch would be close to rebuilding the game from scratch.