It's a rule of thumb and I should have specified that it most particularly applies to borrowing to invest - you should be able to identify the secret sauce that you have (and others do NOT) that allows you to make money where others can't.

A bank won't use a machine to manufacture something, and perhaps you have the skills needed to do so.

But deceiving yourself that you can "borrow cheap, invest it in the stock market, and always come out ahead" - if it was always or even for the most part, why would anyone lend money and not do it themselves?

Because you're missing risk.