It's made so for a fun game play experience, which make pretty obvious sense if you think about it. Predictability is a very boring feature in a TW game.
Rivendell (or the Shire) would be default targets for an evil player if so. Now it's rather intended to work as inspiration for the player, good or evil, to attack settlements and/or facions you otherwise wouldn't to hinder or enable Sauorn's return.
This is not a LotR copy either (then it would be a worthless strategy game since many factions either would not exist or would be very much weakened, units would have to be removed etc etc ; for anyone who wants something like that there's the FotR campaign), this is a 'What if' game at the end of the Third Age.
It's not a far fetched concept either, the Ring indeed had the interest to slip off people's fingers and get to a new sutiable carrier. To claim "But it did not leave Bilbo" is indeed true, but actually it tried, it was just never lucky in those attempts 
Further Denethor and Boromir had no clue about the Ring being found until Boromir was at Elrond's Council. He want there since he and Faramir had dreams that seemed to foretell something about the future and wanted help from Elrond to understand it, since Rivendell was mentioned in the dream. Do not trust the movies to much
Elrond would never been so stupid he summoned folks to 'come have a chat abour the Ring' haha, since then it would be revealed it had been found and its location (here the game chose the movie concept, which is understandable). The 'counci'l of Elrond was a mere chance or a work of fate, since a lot of groups had come to him concerning different matters at the same time as Frodo arrived and Elrond wanted their input when it came to come to a decision on what to do.
@ TLG
These are the scripts for if Gollum or others takes the ring.
It's found in campaign_script.txt in data/world/maps/campaign/imperial_campaign
Apperently the risk is just 2% every turn it will happend, but you can for example change it to 1 or 0.5.
Ofc a very simple change, much more can be done to tweak it, but as I'm no scripter I will not give suggestions on it hehe. If you lower the risk do not forget to open data/world/maps/base and there delete map.rwm before starting a new campaign (not saved game compatible)