It's because the output is testable. If the model outputs a legal opinion or medical advice, a human needs to be looped in to verify that the advice is not batshit insane. Meanwhile, if the output is code, it can be run through a compiler and (unit) tests run to verify that the generated code is cromulent without a human being in the loop for 100% of it, which means the supercomputer can just go off and do it a thing with less supervision.