Matchmaking sistem

Hi. For start..my english is not so good. So ,i have a question if may i. Why the sistem allows and format teams from different divisions on same team? why cant be a game playd only from same division all.? I say that cuz a lot of games are GG from start when a bronze begginr player join in a silver team . The game is ended when he starts lane. Or when a diamond player join silver game with that bronze incognito in opponent team, (GG)

Why that? Srry for this english :D And srry if i posted in wrong topic..