ADC's are a mixed bag. Same with any player or person, really. And it was not Riot that created this meta. It was invented by players in the first season of the game who, through experimentation, discovered the best means of distributing gold and experience among allies.
If you look up the original champion spotlights for season 1 champions, you'll see that Riot actually intended for ADC's to thrive in solo lanes if players wanted.
But the meta we have now became a long-established and expected meta that rarely shifts.
The draft system we have now is a result of the experimental team builder system Riot did away with to ensure optimal queue times for the draft system as well as other modes of play. And the positions displayed are a mere mirror image of the meta that players cemented as the ground rules for playing the game.
Players did eventually cry out against Riot about the lack of bot lane diversity and so, for a single patch, Riot catered to the wishes of those who complained. And, for a short time, an entire class of champions was rendered nearly obsolete with the exception of ADC's whose items were identical to melee bruisers and assassins.
ADC mains complained that their favorite champions were being unfairly shoved out of their home and, again, Riot catered to those who complained and the patch in which we saw bruisers in bot lane was, for the most part, reverted.
Bruiser mains don't want to see ADC's in their lane. ADC mains don't want to see bruisers in their lane. And solo lanes don't want a partner siphoning experience from their efforts.
As for the Support/ADC relationship. They rely on each other to win lane and, hopefully by extension, the game.