In my local package routes are bid every two years.
Retirement, firings, major injuries and death open routes up outside of that timeline.
Considering that it takes about two months for management to make the switch after we all bid, I can't imagine having an annual bid.
And here, if someone with greater seniority than you bids your route, you get bumped.
The only 'lifers' here either have so much seniority that they can't be bumped, or are on a route that no one wants.
Some like to be on their own route, no matter what it's like, and will bid any route just so they have it.
I have six years in, and I just bid cover again - I'd rather move around than spend at least two years on a crap route.
I cover the crap routes, and one benefit of being cover is I know it won't last.