1) Because I don't feel like supporting ADCs that need to hug 100% of the mobs. Your support is a player too.
2) I don't feel like to be totally useless end game because of #1, heck, I seen support go nuts cause I was farming an EMPTY lane while he was dead.
3) I don't want to be blamed for your mistakes (I.E tower dive at level 2 then say stuff like "is your job to support)
4) KS. Seriously, if my auto attack last hits, you are doing something wrong. I am not going to back off at the last second so the enemy hero can flash out or counter gank.
5) The ADC gets all the glory but not support.