Game Engine

Hello, I'm currently in the process of a game development class and was curious as to how you guys decided to developed your game engine. To me it seems like an extension of the Warcraft III and DoTa engine but with less player channeling and more micro movements. But what about the way League is designed made you come up with your own engine? Also what are the benefits of using it instead of other widely used sources (i.e. Unity and Unreal)? Really hope I get to hear back about this because I'm pretty interested in this, and my apologies if these questions are too forward. Thanks!