I just wrote "What's not beautiful?" and I can't get what's wrong with my answer.
Beautiful has a broader and deeper meaning, and I guess they don't want to conflate the two.
For example, if you say "this movie is really beautiful", it means the movie touched you on an emotional level, or its message resonated with you, but if you say "this movie is really pretty", it means you liked the cinematography or found it aesthetically pleasing.
I personally think you can use beautiful when you mean pretty, but not the other way around.
Also, isn't beautiful- 아름다워? I thought pretty and beautiful were different words, although your explanation makes a lot of sense :)
gasp you don't know what Jeopardy is? It's fine I didn't find out until last year, whoops