Why do people play adc?

I used to be an adc main however my love for made me a sup main. I recently played a match as Ashe and for some reason it doesn't feel the same. Why does a full tank 2 shot me? More importantly why does Ashe practically deal no damage? I know Ashe is a weak low tier adc atm but is Ashe the only adc that deals no damage or do all adc's deal less damage then any other role? Also why do tank items make you extremely hard to kill while allowing you to deal a respectable (respectable is an understatement) amount of damage? Should I even play adc or should I just support and move to the apc meta ?