That… doesn’t work for most multi-lane highways. For a 2 lane highway, yes. But you try driving in a city with a 4 lane highway with on ramps and exits all over the right lane and you’ll see why everybody who is not entering or exiting and just wants to go at a constant speed is a problem.
So they cruise in the next lane over. There is still another two lanes for them to pass. Technically, the “fast lane” is just another term for “long term passing,” as in they’re going faster than the other traffic for a long period of time… passing them in the process. They’ll move over when they are able to.
Ever see signs that say “TRUCKS USE RIGHT TWO LANES” and such? There’s a reason it’s not just the rightmost.