The strongest reason that it's bad advice is that existing companies are very hard to quit for most people. The comforts they provide are so compelling that few escape, even when they might quite desperately want to. Existing companies almost(!) completely satisfy Maslow's hierarchy of needs. And beyond simply being compelling, they're extremely consuming, of time and energy.

Just imagine how many more great startups would exist if Google (and others) didn't have 100k of the best developers strapped into The Matrix (:D)