From the matchmaking guide in support: https://support.riotgames.com/hc/en-us/articles/201752954-Matchmaking-Guide
Step 1: Determine strength:
If a player is queued alone, the strength is determined by his personal matchmaking rating (referred to as MMR from here on out) and number of wins for that particular queue. If the player is queued with a pre-made team, the system will adjust the MMR of the team accordingly.** In most cases, we have found pre-made teams out-perform teams composed of unconnected players.**
I understand that Riot has to look at the experience of their player-base as a whole when deciding things like this so, if this is still true, it makes sense to adjust the MMR of the team with the connected players. However, from my personal experience, I never want a pre-made team on my side of the rift ever again.
Why is there no option to say "I never want to have a pre-made team in my team in ranked"?