Why do people not help their healers

I was playing mantis, the problem is that I can never heal anyone because I get instantly targeted by diving heroes, I get erased in less than 5 seconds to getting anywhere near the point, my team then expects heal out of me, I can't heal you when when black panther is shoving his BBC (big black claws) all up in me, DPS, if they have a wolverine, black panther, a spiderman, turn around AND ACTUALLY HELP (also just a little sub rant, why do peni parker players not understand you can stop scarlet witch's ult)