Leage Ranked matchmaking

Me and four other friends qued up for flex. We are in the s4-g5 area and we get matched against three people in plat and two in hight gold. This is not a mmr thing because we have around a 53% winrate. So at the start of the game we start to question why we are qued against plat and gold which have 80+% winrate. Can someone at Riot please tell me why this is a thing? Please and thank you