The two most senior Archmages have held terms for centuries, and life has become impossible to imagine without them. These two have outlived all of their rivals and none would dare challenge their rule. The three least senior Archmages all have held terms for less than 100 years, and their positions are contested by ambitious peers.
1. Gnikllib made a spell that summons him, like one would invoke a particular demon. It must be risky to try it, though - few can imagine an Archmage tolerating being summoned by any spectacular nobody.
|Source: D&D 5e Player's Handbook
One hundred years ago Vemminkainen withdrew from political life. He entrusted the management of his estates within The Hand to his stewards, gave a rather strange speech from the top of his tower, and promptly disappeared. In the history of high elven wizards, this happens from time to time, to nobody's surprise. Everyone assumes that one day he'll make a sudden return with little to no consequence. We're still waiting.