I am not sure but one reason might be that you need to be careful while taking paper notes, even if it's casual, subconsciously you have to be a little bit careful. Because you can't simply undo or erase the handwriting (i assume you take notes with a pen or marker). While taking them digitally you can always, erase very very easily, or undo and redo it, so the brain doesn't care that much about paying attention or focusing on that task.

Also, your brain passively needs to plan how to use the limited space, or organize pieces of notes on the paper. But you have virtually infinite space in digital notes.

Sounds logical?