It's not that complex, they are trying to redeem themselves. They think "well, I've died 5 times, but if I can just get a single kill I can redeem myself." Flawed as the logic is, it is part just being competitive and other part is how hard people come down on people who "feed." In most matches if you go 0/3/0 they say "report such-and-such for feeding" they after awhile you think "I might actually get in trouble."
But above all, lets be honest, it really doesn't feel good to get carried. You want to be a contribution to your team.
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I don't know why but i've seen them.
I was duoing on my smurf with a bronze 5 friend, was 11-0-1 in lane as zed, and a team fight broke out near dragon. and our top lane for some reason decided to TP top to clear a wave DURING the fight, and our ADC Lucian went to farm bot because "there was a huge wave", leaving me, and my duo jungler(granted he isn't very good), and the support in a 3v5.
The 3 of us died after i killed 3 of them, then the Lucian decided that "OOPS I should go clean up" and went 1v2 and for some reason died without killing either one......
it was a nightmare.
That is just poor map awareness, and you can't really blame a bronze 5 for not knowing what is going on. A big part of why B5s never place wards is because nobody is looking at their map anyway.