I think its a technical issue that's hard to deal with, since in many games, pathing and collision has been a problem.
I don't know what it is, I'm not tech savvy, but its just an observation.
But I think part of it makes sense. Sure, its a bit annoying when you are stuck in a creepwave and you're having a spasm. But say in a movie, there's a chase scene, you won't see the guys run through a huge crowd of people when there's a much emptier space next to them.
Minions do act as a bit of a block as is, since attacking champions when you are next to a massive creepwave will get you aggro.
If there are ways to make the mechanism cleaner then by all means, but I think its not too much of a problem.
Besides, why else would we have Fizz passive?