Fill In Ranked

Does anybody have a good reason as to why you can get put in a game as a role not of the two selected in ranked? I really don't have the urge or the time to learn how to play support, and for some reason I have been getting forced into it a lot lately which forces me to dodge and wait an extra 5 minutes to play a game. I totally get why this is a feature in unranked, but with competitive motive behind the game I would very much prefer to wait an extra 2 or 3 minutes to get mid or jungle as opposed to being forced into a game that could last an hour playing something I have no desire to play as well as probably doing bad.