> Unity should've spun their own language/runtime

They did, and that's why their C# API is such an oddball. Unity used to support 3 .NET languages: UnityScript, Boo, and C#. UnityScript started as the recommended one, but I believe it was just a JS-like syntax for Boo's semantics. Eventually C# users dominated, and UnityScript and Boo got deprecated and removed, but Unity's .NET API was left with all the quirks from their UnityScript era.