I think there are many of examples out there; the simplistic answer to the OP question is "we do; you just haven't heard of them." My favorite is OmniGraffle..

I think there are many of examples out there; the simplistic answer to the OP question is "we do; you just haven't heard of them." My favorite is OmniGraffle... you can enter nodes in a text-entry outline style mode and the results are updated instantly. The default styles and stencils, while recognizable as being generated by OG, are inoffensive, and the layout tools are among the best of any layout or presentation software. Others have mentioned GraphViz, which is a general purpose graph description utility. Outliners as a general category of software have existed since the 1980s and seen an ebb and flow of popularity. Just a few off the time of my head... Think, More, Acta, OmniOutliner. The Frontier scripting language in particular stands out as a programming environment completely integrated into an outliner paradigm. My first exposure to code folding came when discovering Frontier sometime in the mid-'90s. As Apple put its Mac scripting focus on Applescript, Userland reimagined Frontier as a web site programming environment: templating, scripting, and content management all rolled into one. But at the same time it was a general purpose scripting environment. Maybe my graphing needs are not so deep that these tools (or some combination with a few others) wouldn't suffice for a variety of tree/graph related tasks.