Matchmaking making me crazy

I play everyday and it’s hard to find a fire team to run with regularly,so I end up playing a lot of swat and slayer with 3 random. My rank has never been higher than plat 6 in both playlists but I have played with diamond and even champion level players. When I load up alone I may get 2 or 3 truly balanced matches and lead my team in kills. Then the next match will almost always be a team of diamonds or higher and I get wrecked
Go from 20 and 8 per match to 8 and 20,from having a good time to wanting to never play again. Why does matchmaking continuously place me against players who’s skill level is so cleary far above my own?