I believe the mechanism, at its core, is that engineering or anything technical is to some degree a competitive field (not as in pay, but as in having clear performance differences between who is good and who is bad). This means any newcomer will be a reason for the team to flex and prove that they don't need the consultant. Similarly, the consultant is trying to show just how much of an absolute maven he or she is.

There is an element of truth in what you say. And yet, when people actually behave that way, it often becomes toxic, which ironically makes them worth much less as either employees or consultants.