> How would you represent a tree or graph data structure in text to a human and let them manipulate it?

Tree:

    '(root (left (left-left left-right)) (right (right-left right-right)))

Graph:

    '(graph (#1=node1 #2=node2) (#2# #3=node3) (#3# #1#))

Solved problems since at least the early 90s.

> Yet using it to paint is a lot harder.

Sure, but drawing is only one activity among many possible activities.

> As a software developer you're often manipulating only text.

I think that's true for most users: they are reading articles (text); writing comments (text); sending social updates (rich text); watching videos (not text); playing games (not text, but may have textual content, e.g. in-game chat).