First, this is not strictly true; many senior ICs get paid more than managers, and the comparison between manager-track job levels and IC-track job levels varies at different companies.

Part of it is that good managers act as an umbrella for their reports and keep unnecessarily distracting or stressful issues from taking up their time. But that means the manager is dealing with that stress instead.

Part of it is that managers often have a broader scope of responsibility. Senior managers and higher often have multiple projects under their belt and are held responsible for their continued success.

Part of it is that the success state of a manager's work depends on the success of other employees, which makes it more difficult for them to control whether they do well by their own effort. A really good manager could be paired up with a really bad employee who eventually has to be fired; did that manager do a bad job since their report got fired, or did they do a good job identifying that they needed to be fired? It's situational, and that ambiguity increases the risk of being fairly rewarded. Higher risk demands higher compensation.

Part of it is that demand for good managers is high enough that the market prices their salaries higher. Anecdotally, I can say that a bad manager hire has a much worse impact than a bad IC hire, so the stakes are higher, which raises prices.