MMR

Hey I'm just asking something, why does the MMR (Match Making Rating) have to be a thing? me and my friends play a lot we're "Casual player" or what ever the high ranking people call the players that never play rank or almost never do and spend all their time on normals. because we never played rank we all let our rank fall to bronze in the last season. we obviously set our ranks for season 5 (And how it was set up this year we all got some where in bronze) we decided "Screw it, lets climb" and we started to in ranked 5v5 games, but the thing is because of our MMR we are getting placed with gold and higher every game very rarely being matched with silvers. I'm not saying "Oh were just that good in normals" I'll be the first to admit I am the worst at this game, that's just how it has ended up. during these ranked games of team 5v5 we get placed with people of much higher ranking, the most resent game we played we had a plat player on the other end surrounded by golds, and this hasn't been the first time we have fought a plat. because of this I'm just wondering, why does MMR put bronze players with plats? I know that its to put together players of the same skill lv but there is a large knowledge difference from bronze 3 to plat 2.