They aren’t optimising for speed, sharing, or searchability. Notes of this type aren’t an inadequate database, but a tool for thinking. Note-takers visualize and symbolise their thinking while they work through a problem. Writing and drawing reduce cognitive load and externalise information so it doesn’t have to be held in memory. Notes also create a useful record of that process so you can see how you got to your solution. Notebooks are just a convenient UI for that process.