1. You can cut, tear, fold, roll, crumple, burn, sew, staple, glue, laminate, etc. it, including sticking found objects to it, wrapping things with it, making it into 3-dimensional structures, ...
2. It comes in a wide variety of colors, textures, thicknesses, and sizes – if you want you can cover your whole wall with one piece of paper. Some paper is translucent and can be used for tracing.
3. You can draw/write/paint over it with a wide variety of substances with a wide variety of appearances. You can fit visible information more densely on paper than pretty much any other available medium.
4. There are many nice tools for interacting with paper, such as rulers, compasses, french curves, pantographs, planimeters, ... You never run into a case where two drawing tools can’t be used together because they run different operating systems.
5. You can easily make physical copies of electronic documents and images onto paper, easily manipulate paper documents, easily copy paper documents from one piece of paper to another, and easily put paper documents back into digital form.
6. It’s incredibly cheap.
7. The right kinds of paper stored properly last a really really long time (centuries if not millennia).
8. It is light and portable, can be stuck up pretty much anywhere.
9. It requires no batteries, no legacy systems, no software updates, no expensive equipment, no special training, ....
10. It is its own concrete physical record, which can be effectively checked for tampering and made difficult to forge at scale. (Hence use for money, ballots, contracts, titles, ...)