I bet everyone on here uses Word (or some other word processor) and Excel (or some other spreadsheet). They are just so incredibly versatile that they encourage creativity in a way that most specialised software doesn't.
Both Excel and Word can be used for creating: books, letters, posters, tabular data, lists, art, project planning, first aid logs, business applications, games, and much much more. I bet if your favourite list manager went away, you could reproduce it in Word or Excel.
I think the reason they are so ubiquitous is that they provide minimal structure and a set of useful and fun features. They then let you do what you want without further constraints. Word gives you a page and allows you to put text and other things on it. Excel gives you a grid and allows you to put things in that grid or on top of that grid.
Paper is similar but even more powerful. You get a sheet of flat stuff and you can do whatever you want with it. You can
- write
- draw
- rip (there is at least one video of David Allen showing how he tears paper as part of his processing)
- scrunch
- fold
- combine with other pieces of paper
- spread out all over the desk or floor
- pin to the wall
- attach stickers
If you need a little more structure, you can buy paper pre-prepared with lines, dots, squares, holes, crosswords, colouring pictures and prose.
You can write books, letter, tabular data, lists and all the rest of the stuff you can do in Word and Excel too.
I often miss the paper system that I started with.