Which kinda makes sense, because a non-existing button usually isn't clickable, and also wouldn't be able to emit events in an event-driven API ;)

Btw, the drawing doesn't happen in the button function, it just tells the UI system that there exists a button with the label "Start" in the current frame. The rendering usually happens much later after the UI system has collected the entire UI description for the frame, otherwise rendering would be very inefficient.