Why is "quanto" necessary here?

Why di quanto and not che?

Why is 'È più difficile di quanto tu pensi' not accepted? The only difference I can see is the 'tu'.

I need assistance with this one please. Why does "che" not work here? I see two people asked this question years ago and nothing from DL. :-/

If you look at the hints for "than", di quanto is one of them. It's just how you form the sentence.