Because it's a problem with a 50 year old UX design that we still use today and aren't changing any time soon. WYSIWYG UX is easy to use, because you can simply train the users to manipulate what they see until it visually represents the output they want. This is the predominant way we've trained users for most of the history of GUI software.

But, WYSIWYG on a 2D screen is just an inherently leaky UX abstraction when dealing with 2.5D content, e.g. layers.