Toxic mercy and DPS duo

I got into a comp match and I wanted to play Mercy so I picked her as my preferred hero and the other support did as well when we got into the game I got mercy first and the mercy‘s DPS duo said we’re throwing so they went to venture and went into their back line over an over again The next round comes along. I let them have mercy and then the DPS duo starts trying again. Why are duos so toxic? especially in the mercy community. Ps I reported them for throwing already. (This was in high diamond masters rank.)