I was thinking more of the machine that creates the dice for the sake of freshness, for one. Further,I can keep an onion for weeks, but a chopped onion has a shorter shelf life.
For that matter, there are food processors that do a terrible but passable job at the prep, but again, the cleaning is suboptimal.
Um. You can buy most vegetables pre-chopped. They cost less than Soylent. They sell them at Walgreen's!