It definitely isn't the most used how on earth did you come to that conclusion? It's increasingly popular definitely and it is a good engine but it's not the most used.

Unreal Engine 5 is a relatively new product and Unreal Engine 4 was basically on par with the Unity, so there wasn't a great reason to use it over unity especially considering the Unity had the developer community behind it. Even now Unity still has more community drop-ins than Unreal.