Plain text
Plain text is text which is easy to parse as a human, as opposed to code that is mainly supposed to be readable by a machine, such as executable binaries (.exe, .com, elf) or image file formats (.png, .jpg .jxl, .avif, .webp). Inbetween those defintions there is markup and programming languages; those are trying to be readable for machines and humans alike.
The freedom aspect
Text files offer more freedom than formats like videos due to them being easier to modify, share, extend, and read. Viewing videos is limited to a sophisticated software and hardware stack requiring plenty of resources, which applies to the editing process tenfold. Meanwhile text files can be viewed on a ZX Spectrum or PDP-11 without any issues whatsoever.
Not only do plain text files require less resources and lower over-all code complexity to even be processed, they also are easy to modify. Editing Markdown with vi(1)/ed(1) is less demanding for both the computer and the editing human, as it doesn’t require an elaborate software setup like Sony Vegas. Have you ever tried adding an additional thing in a video? Even if you didn’t, intuitively it’s clear that this process demands more time and resources than simply editing a Markdown file.
This, by the way, is also a good argument against those “graphical”/GUI programming languages: Programming a game in Unreal Engine’s GUI language is not just less free (as in freedom) because it presupposes the Unreal Engine, the fact that this language presupposes a binary format which only a single program in the world can edit, as opposed to a textual programming languages, makes it less free.
The efficency aspect
A text file will always have more information density than a video essay with the same words, calculating given information per MByte, i.e. information for human for information for the computer.