How do you get to that conclusion? I find that if I have a text editor, I can write my thoughts down and then visually put them in order or encase them in a more general concept with ease, which I couldn't do when writing on paper.
But I've noticed that many many many people report the same effect, that there's something about pen-and-paper writing that's more effective for thought-lubrication. I resisted for a long time, but now I too am a convert to this school of thought.
Similarly almost everyone notices the downside: it's easier to reorder, reorganize, cross-link, etc, those thoughts, in a text editor (to say nothing of more sophisticated software tools). Some people have systems for doing complicated things with paper that they say mitigates this downside, but I am not currently one of them.
I guess it's possible that your brain just doesn't have this pattern in it. (That is, the pattern of finding pen-and-paper more effective for getting the thoughts to flow.) I mean, for all I know, maybe the huge silent majority doesn't have this pattern.
But writing enables arrows, lines, crossing-out, small-text notes, circling, variable pressure, colour, etc. Richer than ordering/indenting text. Also higher contrast.
Some of it may be just ... overload your brain so it cannot think of anything else ..so you stay focussed for lack of choice.
In my personal experience, a helpful feature of pen and paper is that it is less effective than keyboards, it takes me more time and focus to write things down. Maybe this gives the rest of my brain more time to catch up and understand the things that I am writing.
Written text is also less efficient when it comes to searching. This forces me to organize my thoughts better because I know I won't be able to CTRL+F random keywords later.
I don't write code on paper for (probably) obvious reasons, and I tend to write essays in a text editor although I also enjoy the act of writing on paper in that situation.
Personally, I find while the computer provides powerful writing tools, it also provides powerful distractions.
Maybe you get notifications you just quickly want to check, that slack message from the boss could be urgent. Maybe you decide to just check you're using that obscure word right, or to research a detail for your writing and an hour you haven't written the number of pages you set as your goal. Or maybe sitting in your netflix-watching chair looking at your netflix-watching screen just doesn't put you in the right mindset.
I remember my mother once doing the opposite: she wrote a long letter on the computer, so she could edit and re-write until she was happy with it, then printed it out and copied it by hand, for the personal touch of a hand-written letter (a peacemaking letter to a relative).