Why are some things (like Vector2Field) not in the UnityCsReference repo, but some others are?

If you look in https://github.com/Unity-Technologies/UnityCsReference/tree/master/Editor/Mono/UIElements/Controls, you can see entries for things like the BoundsField or the ColorField. But if you look for some other things, like the UnityEditor.UIElements.Vector2Field, it's not there.

Am I missing something? Or why is the source-code for Vector2Field (among many others) not available, but other things in the same namespace are?

 

Thank you for your reply. I didn't consider this as this behaviour is not common Unity (because of MonoBehaviours having to be in seperate files). I appreciate the help!