No one makes it on their own. Someone is guiding them, telling them the unwritten rules, preventing them from making grievous errors, supporting them at uncertain times. Institutions (departments, schools, universities) take responsibility for those they are promoting, making sure that those they back have done the right things and the institution is willing to bet its reputation on that person. (Hence, if the institution is unsure of that bet, it might not be willing to place the bet. Risk is not the problem; uncertainty or lack of information is a problem.)

You would not want the professor of surgery to promote a candidate who is not first-rate as a surgeon-- It's more general, you would not want a university department to promote a candidate who is not first-rate as a scholar. Would you trust your own children or spouse to this person's care and tuition?

"Surgeon" or "scholar" can be slippery notions. The candidate might be an excellent performer but not a fine researcher. The candidate might be a superior manager, but you do not want them actually teaching or dealing with patients or doing research. Institutions will make judgments of this sort all the time--few people are superior at everything, and only some strengths are relevant to that institution's mission.

We make mistakes, probably at least 1/3 the time, surely no less than 1/10 the time. I believe we can do better, but that would demand that we are honest with ourselves, ask tough questions, and be realistic about our judgments. And if we have made a mistake, how can we rectify them through mentoring, guidance, and incentives.

In the background is the fact that the performance of members of a department may be rising in time, new members in general stronger than current ones (at least in mean). If we realize this, when we make appointments we don't want to create a permanent underclass, so to speak, by making another mistake.