They're either looking for a purple squirrel (like trying to hire the original developer of the technology), trying to move someone internally into a position that requires them to also seek outside candidates (by making the requirements impossible to fulfill), or dev told HR they needed someone with "X" experience for a senior role, and HR translated that to "10 years experience with X" (X having been out for 2 years).

There are three of the standard answers that come up any time this topic does.