Why? Unreal Engine has targeted iOS + Android phones for 5+ years now, successfully. (Fortnite, PUBG, Dungeon Defenders, Horn, etc).
More mobile developers are experienced with Unity over Unreal today, but that's largely inertia more than anything else.
F2P mobile dev is also very UI heavy, and UI work with Unity/C# is easier than dealing with C++/Unreal.
And Unity has a wealth of ready made plugins that can be quickly integrated into a project for things like IAPs, advertising, analytics, etc. Although maybe UE is catching up in that area by now?
Dungeon Defenders and Horn are not UE4 titles, UE3 isn't available anymore. PUBG and Fortnite only work on higher-end phones. UE4 is going the "easy" route by not supporting older devices. With Unity, you could still target fixed-function hardware up until recently.