> It would make sense, and all text in a user interface should be user selectable and offer the same behaviors.

How would you know? You have never used such a system because it doesn't exist, how do you know it's better?

If you had ever implemented something like a drag and drop UI, a context menu, or even a a simple button using plain old HTML, you would've immediately noticed that selectable text interferes with user interaction. It's the first thing you disable and it's the reason why "user-select:none" must exist.