Why do adcs blame us supports for being useless?
Because, the ones that do it, are jerks that only care about their kda and not the team. They'd rather have a lot of farm and kills with no deaths. And if they die because they stupidly went in without the support, then they needed to blame someone other than themselves. Because hey, the support is there only for the adc.
It's the same for asshole supports. It's your fault if you die, but they'll bitch at you for it, even if they were right there and didn't bother to help. They wont want to die, because their kda revolves around deaths and assists over kills, but they wont want to help either because they might die and ruin their precious 0/0/1 kda.
It's a two-way street with the toxic adcs and supports. Just remember that not all of them are like that. You can get a great, nice support and a great, nice adc.