We don't have fixed terms for mods. This means that if a mod goes off the rails or loses sight of their role they remain in a position of power and can enforce their personal opinions or agendas on the site irrespective of the site users.
I think this is a bad idea, there should be a fixed term as a check and balance just as most other facets of lives need checks and balances. If they're doing a great or even mediocre job they'll probably be voted back in, if they're actively detrimental or can't be bothered yet won't quietly remove themselves, then the site users have at least an option of removing them.
This is a common check and balance in both work and volunteer situations.
On top of the check and balance scenario currently we have a situation where mods are mods forever, so even if they are inactive they still take up a mod spot.