First thought: Xamarin is now open source—great! Second thought: Every time I looked into Xamarin (and I do this every 6 months) there was a lack of killer apps created with Xamarin on both iOS and Android, users' experiences with Xamarin were either rare or negative and in total the community felt non-existent Third thought: Ok got it, they open source to get PR, build a community; I hope that helps and it's not the…
Couple billion dollars in revenue generated with it via Unity alone. https://en.wikipedia.org/wiki/Unity_(game_engine) -- and that was a HORRIBLE version of it. Way better now.
Xamarin and Unity don't have anything in common, other than the fact that Unity uses C# for scripting (on an ancient runtime).
Cross-platform app development frameworks are generally awful because they don't tightly integrate with the native look and feel of every platform. Video games don't even try to achieve native looks on any platforms. They're always full screen works of art, without any hint of a UIButton (iOS) or TextView (Android).