Because some do not have auto-completion.

[Gama11]

It explicitly talks about this:

Note: if you're using FlashDevelop, it should automatically create an import for FlxButton (import flixel.ui.FlxButton;) at the top of the class. This should be mostly automatic whenever you use class, but if it doesn't add it for some reason, you can enter it manually, or, highlight FlxButton in the editor and hit Ctrl+Shift+1 to add it.