So let's say you have a 50% winrate, you coinflips games on your main role, and aren't too bad. You're in an elo that you belong.
Then you're autofilled and rather than getting teammates that are aggregate 50% winrate(the ideal), you get 55% winrate teammates. It's actually BETTER for you to be autofilled than to be playing your main role. All you have to do is not be the worst person on the map and most times you'll win(albeit slightly). It also means that when people are climbing, playing well, and have a good winrate, they will be more likely to be matchmade with autofilled teammates. I don't think anyone wants that.
Riot has said they are planning to work on autofill this year, and I am willing to bet that the primary hurdle is finding a sufficient number of players to match everyone any better than they already are. There is a huge playerbase, but if 10 million people were playing League 24 hours a day, your matchmaking would be very fine tuned. But especially above Gold, where you have a tiny subset of players and LP decay keeping it tiny, you are, to some extent, relegated to playing with whoever is in your region at the moment.
I have to say, matchmaking isn't THAT bad. If you've never been Iron 4/Bronze 5, you haven't seen what kind of imbalanced matchmaking is possible.
If you're autofilled, you can check your teammates winrates in lobby and dodge 1/day if they dont look like anyone's gonna carry you. Heck, being autofilled support with the draven one trick who is on an 8 game loss streak and is now playing Aphelios for the first time...probably not a win either, no matter who your teammates are.