First line managers rarely have the power to simply fire on a whim. There has to be cause, and it needs to be evidenced, and there will be multiple people involved in the decision.

If you're a more senior engineer, you may report to a manager of managers, and line managers may be your peers organisationally, and probably not paid as well as you are.

Engineers also get to make a different kind of decision to managers; and it's often the longer term, higher leverage, more strategic kind of decision. Line managers are much more operationally focused and rarely have the bandwidth or tasking for strategic decisions.