why do people try to 1v2+

I just lost a game we were recking because our adc decided I have 13 kills i cna 2v1 now. the funniest was when he decided to 1v4... They refused to listen to me because they would jump into a fight nilly willy without me. every time I tried to group with them they would be dead before I got there. Only reason I wasn't with them was because they were dead so I would go do something useful while they were dead like keep lanes pushed, and then repeat...