The problem is that Support dictates the bot lane, so if you are a competent ADC but your support has no clue what to do, then you basically lose your lane phase because of it. It's even worse when you are a lane winning ADC/support.
I had a game where the enemy Thresh was roaming around the map while my Leona stood behind me letting the solo Caitlyn free farm. Like, zone her. She's alone. Just walk up into the bush and throw abilities at her if she dares get near the wave. Boom, now Cait is behind and will be extremely worthless during her mid game. Or, go counter Thresh's roams and stop leeching exp.
Or, you have supports like the "I'll stand 9 feet behind you and shield" Janna's who take Spell thief without using a single ability on the enemies so you're essentially in a 1v2 with 200 extra hp now and then. Great...
Or the "I don't want to actually support" Veigar's that miss every Q and cage (like not even trapping them inside it, just straight up misses), gets fed up because you aren't playing aggressively enough when he misses, starts taking farm, pushes the wave, runs forward after you start backing, dies to the obvious jungler who's been in that bush on a ward for almost 2 minutes now, and then blames you for leaving him.
At least with an incompetent ADC I can at least use them as bait cause the enemy team will think "we have to kill that ADC" so maybe we can get return kills on our actually fed members. Or, if all else fails, hope the ADC knows how to stand still and right click late game and pray the enemy team doesn't notice that they got their 4 items and now actually do damage despite not knowing what to do.
Plus there is always the age old adage of "you used all that to kill a support, lul".