They have it disabled because you shouldn’t be using it. The engine core and its modules are built with RTTI off. Even if you enable it for your module, any dynamic cast that treads into engine code or any of its plugins == undefined behavior. Unreal has provided/recommended safe alternatives to dynamic casts for a reason. Because they don’t want users playing russian roulette with millions of lines of engine code.
If you need dynamic cast behavior, you should be using Cast<> for UObjects or enum/type flags for pure C++.