Most existing languages weren’t written specifically for making games, let alone integrate fully with UE. To be useful, they would need to be changed so much that they would no longer conform to whatever standard defines them (which also might cause issues with whoever defines that standard).
Sometimes it really is better to reinvent the wheel, if you’re trying to do something that wheels weren’t designed for.
Fortunately, Epic just bought a company that created a language that was written specifically for creating games and can integrate with UE4.