Thats because 95% of player base is on rank bronze ... and SE makes it that way that you need 51%+ winrate ratio to pass bronze rank, which is maybe OK for premade 4v4 but not 4v4 solo. Therefore everybody is sitting on bronze, so thats why you are getting paired with bad players afkers and so on.

Solution is to spread more players across ranks regarding their skill. For example:
Bronze to Silver... 30%+ winrate needed (+25win / -12loose)
Silver to Gold .... 40%+ winrate needed (+25win / -18loose)
Gold to Platinum .. 50%+ winrate needed (+25win / -25 loose)

We have 5 ranks in game, so why not use them?