Why do people purposefully get placed in lower ELOs after their placement matches?

You have to go 8-2 to even get placed where you ended the last season. I ended in Silver 4, (played less than 100 games, so I would've been much higher.) and I already lost my first placement game, AND I ended that game with a negative KDA since I had to go tank because our renekton wanted to splitpush (IKR!). I've been told that the first game has the most influence on your placement and the last game has the least, so now my fate is sealed to be bronze if I play my games out. No offense to bronze players, but last game I played against bronze players I had 300 ping 80% of the game and ended the game 9-0-4. I would be very angry, and probably toxic if I had to deal with them on my team (which I do on a daily basis since my normal mmr is trash from playing with goofy 5 man groups) in a ranked game. So my question to you is, Why would you purposefully place good players with bad ones? Wouldn't that just create more toxicity?