As someone who daily sends messages to myself as a form of notetaking, for everything from todos and shoplists to ideas and bookmarks, I've thought about this
a lot. The only conclusions I came to are that I keep using messages because (1) they are convinient and synced across devices, (2) app is already installed and frequently opened so it rarely needs to load up making notetaking faster, and (3) I do not have to think about organization.
I believe the last point is most important, and is where your app is misaligned with my reasons for using messages - you seem to have spearate folders and notes, prompting me to think "where does this go?" before writing, which is something I really want to avoid.