"Paint apps use brushes and canvases"

No, they use skeuomorphic simulations of those things.

(edit, to expand a bit)

We have the capability to put anything on the screen that we can imagine, but we're not doing that. We're limiting ourselves to bad simulations of real-world tools. The "brushes" are just more of the same thinking that gave us "desktops", "file folders" and "trash cans". As others have noted, those things do make it easier for newbies to learn the system, and yeah, that's a point in its favor. It's just disappointing to me that we haven't come further than we have with respect to truly innovative interfaces.

You even see it in VR! You get the same menus, buttons, and whatnot, but, hey, now the components are rendered as high-resolution stereo images. Yay! I guess?