They don't use ratings to seed players into the world championship (well, not for the most part, I think 1 player gets in that way). To get a shot at the world championship you have to win several tournaments like the world cup, the grand prix, and of course the candidate's tournament.
Ratings mainly serve two purposes:
1) To measure relative skill. This means individual progress as well as comparing players.
2) To make better pairings in tournaments and online.
This makes no sense . if we did not have ratings we would not know how strong we were in comparison to players all over the world. Sorry but this idea is nonsense.
And there used to be matches to determine the challenger for the world championship. They were called candidates matches. First you'd win big tournaments (zonals and interzonals) then the top players would be put in a bracket and play each other in elimination matches. The winner of that would be the challenger. These days it's just the candidates tournament, but to get into that has extremely difficult requirements like winning the world cup.
Last cycle MVL was eliminated from the world cup without losing a single game.
As a result, he wasn't invited to the candidates tournament even though he was one of the top 5 players in the world.
As another example, Nakamura has been a top 10 player regularly for the last 10 (or more) years.
However IIRC he's only played ONE candidates tournament.
In other words the requirements are extremely challenging, and not purely based on rating.
No other sport uses mathematical ratings to determine much of anything. The only example I can think of is the NCAA basketball tournament, where the committee uses RPI numbers as a consideration of how strong the teams are, but use a number of other factors as well. So why doesn't chess? You could basically group grandmasters into divisions based on Geography - North America, Europe, Russia, etc. Everyone plays each other in these divisions and then the winners (or top 2, 3, etc) of those go the playoffs where they eliminate each other until going to a world championship. Makes sense right?
edit - you could have a system of promotion and demotion like soccer. So the grandmasters with the lowest score get demoted for the next cycle, while the grandmasters who did well in a lower section would get bumped up for the next cycle.