That's because it would give more opportunities to argue when the year is up. While there would be some benefit to clearing out the deadwood, this serves no useful purpose in relation to active admins.
Assuming terms of one year, the idiots would make themselves obvious long before that. Those who use admin privileges sparingly but correctly, thus consistently flying under the radar, would be unjustifiably forced annually to run the gauntlet of the inhabitants at Requests for Adminship.
The first thing that I would scrap about adminships is the voting. Establish a few easily defined criteria for becoming an admin, and anyone who has met those criteria and wants to be an admin then only needs to ask and it will be granted. This simplicity would bolster the principle that adminship is no big deal. It would also be no big deal to de-sysop for absenteeism, because that person would only need to become active again in order to be reappointed.
Repeated incivility (however we choose to define that) could be a further basis for automatic desysopping, but that too can be reversed when the person (who can still make ordinary edits) shows an ability to get along with people.
Ec
Parker Peters wrote:
I seem to remember proposals for limited-term adminship on here at some point, but it was a while back and got shouted down pretty quick...
I mean, there's more than one way to do it. On Meta, adminship is for a year at a time, for example. On sr:, admins aren't just janitors but have editorial decision powers. Etc.