I have no experience with graphic design programs, but I would guess that it has something to do with the fact that VSTs are self contained and have extremely s

I have no experience with graphic design programs, but I would guess that it has something to do with the fact that VSTs are self contained and have extremely simple interfaces. A VST takes some input (MIDI or audio) and produces some output (MIDI or audio). That's it. They're extremely modular, and you can chain them together in arbitrary ways so long as the inputs/outputs line up. I imagine it's not so simple in the graphic design world, and without such a simple interface that everyone can agree on, it's much harder to create standardized plugins that everyone can use.