"Bad" is vague, subjective moralist judgement. It's also easily manipulated and distorted to justify firing competent people who did no wrong.
> It’s pretty obvious when someone starts actually working if they’re going to a net positive. On the order of weeks, not months.
I feel your opinion is rather simplistic and ungrounded. Only the most egregious cases are rendered apparent in a few weeks worth of work. In software engineering positions, you don't have the chance to let your talents shine through in the span of a few weeks. The cases where incompetence is rendered obvious in the span of a few weeks actually spells gross failures in the whole hiring process, which failed to verify that the candidate failed to even meet the hiring bar.
> (...) might as well probationally hire the whoever passes the initial sniff test.
This is a colossal mistake, and one which disrupts a company's operations and the candidates' lives. Moreover, it has a chilling effect on the whole workforce because no one wants to work for a company ran by sociopaths that toy with people's lives and livelihood as if it was nothing.