Why if you primarily pick fill you cant secondly pick a lane you don't want?

This is just a question, i was wondering why when you're playing ranked you can't pick fill and then guarantee that you don't one get one role. I personally think it would be a cool idea because then you get either 1 of 4 lanes and you would still find games reasonably. E.g. I pick fill for my primary and then not top for my secondary.