Why it is so hard to create outline shaders for 3D objects?

All these toon shaders I have been seeing render an duplicated mesh behind it and extrude the normals of the vectors to make it look like it has a border, but it feels a bit hacky to me. (Also won't this affect the preformance because you have to render the object twice?)

Basically I am trying to create an outline shader for a plane so these toon shaders do not really work.

It's probably due to my lack of deep knowledge about shaders in particular but I am really confused and would like someone to explain or direct me towards an explained answer.