As many other people have said, ADCs and Supports have the least amount of agency with their laning phase; its all bot lane parties either from the Jungler or the mid or both. Their lane isn't decided by them and even if they happen to do well, it doesn't really fucking matter if Top lane has fed a bruiser.
That said, why people pick ADC still couldn't possibly be because they enjoy playing ADC. >_>; That's like asking,"Why do people pick Assassins instead of mages?! AT LEAST IF THEY FED ON A MAGE, THEY'D STILL HAVE UTILITY AND BE USEFUL!" or like asking,"Why do people pick Bruisers?! At least if they were a tank, they'd have CC!"
The short answer is because they don't want to. They want to play what they want to play since this is kind of like, Iunno, a game? That people play to have fun? What a novel idea!!!
And this doesn't even equate into Mages =/= ADCs. The two playstyles are vastly different and have next to nothing in common. One is spell based, usually has skillshots, and usually serves a far different purpose and the other is AA based with limited skillshots. At least most Assassins can be seen as similar to Mages in that they're caster based and combo reliant. Or bruisers can be seen as similar to tanks because they're beefy and usually can engage a fight and cause disruption. Mages and ADCs play nothing alike; why would an ADC main pick an mage? Do you want them to feed worse?