I seriously don’t understand matchmaking

Seriously, I’m a high diamond/masters player who has been on the line between masters and diamond for the past week or so. I flip flop between low diamond/borderline plat games where no one joins chat and everyone just runs in and dies, and actually good masters games where we communicate and work together. It gets so frustrating trying to carry a team when they don’t care about winning and just want to run in and die. Like one game I’ll go from playing with and against top500s, to being with 3000 players no where close to my sr. I just don’t get it