Well, physicality is part of the point: it slows the writer down and provides more engagement with text (with the text being read; and the text representing the reader's ideas about the text being read; and the text representing the reader's ideas about text representing the reader's (prior) ideas about the text being read). Furthermore, handling, pushing around, and organizing actual paper on an actual desktop has (in my experience) effects on the mind that are both different in kind and superior in degree to those achieved by merely thinking, speaking, or typing about something.

The availability and simplicity are part of the point too: pencils and paper are ubiquitous; and easily found and inexpensively purchased if not immediately available. Written jots and drafts of ideas, summaries, and reactions (of the kind we're talking about here) are easy and quick to make; they readily reveal their content; and they're easy and quick to view simultaneously, to rearrange, to stack, reorder, organize, store, and share.

Compare voice recorders: the devices are not ubiquitous and are costly; they break and are costly to repair or replace; they have many dependencies (batteries/charging/charging plugs; switches prone to failure; firmware; drivers; USB plugs or adapters that can become missing, damaged, or obsolete themselves); etc.

And compare the audio tapes or audio files: they don't really and fully reveal their content by glancing at them; they're hard to keep track of; and they're more difficult to change or reorganize.

All that said, I think voice recorders have a place and I use a dictaphone myself on occasion — but primarily to spit out several ideas quickly when time is of the essence, I'm afraid I'll lose the thoughts, or I can't find the words to describe them in writing but somehow can blab about the gist of them to create a sort of bookmark for later processing.

But then I have to remember to check to see if I have audio files of orally recorded thoughts to process and then either transfer the files to my computer for transcription and printing, or else listen to the audio files and write the thoughts down myself. <Inevitably: oops, I forgot I had these; oops, device isn't here or isn't charged when I need it; oops, Wi-Fi is on the fritz; oops, I want to reorder, reword, or cross out some of these thoughts but not others, and I can't remember or retain in working memory the whole of my spoken audio to restate it all from scratch; etc.>

In a way, doing it by hand avoids the same type of virtualization problems I experience when budgeting and spending and tracking money: doing it physically instead of by computer keeps or makes it "more real" to my mind, and I seem to make better decisions and fewer errors. That benefit alone more than makes up for any time supposedly lost in doing it by hand: slow is smooth, and smooth is fast.

-- Later Edit ---

The slower pace of writing by hand is also a big help because it acts as a filter. I find it helps me to filter out low-value reading (because I easily perceive when analyzing, summarizing, and reacting to something in handwriting would not be a fun or beneficial use of my time). And I find that it helps me filter out my own thoughts (because writing out the less-important ones would similarly take too great a share of the time and energy I have to spend).