Wait so why do ranged ADCs get their personal wardslaves again?

A. Supports aren't ward slaves, they serve multiple purposes, warding is only one of them. Every team member, including the ADC, should contribute to ward coverage.

B. Melee ADC' have been seen bot lane before, just saying.

Because melees aren't going to carry anyways.

Because ranged champions have better late games than melee champions due to last whisper/void staff and the fact that Riot tries to balance 400-500% more range with like 20% better base stats

Ranged adc's scale mostly with gold, they generally have a support to make sure they can stay in lane and continue farming. Since they are in lane more often they are better for bot lane to maintain Dragon control early. Most mages and bruisers scale better with level so they tend to go into solo lanes to get better xp along with farm(with of course some exceptions to try and counterpick that). You have AP characters generally having good burst damage and roaming potential, so they are seen mid lane so that they can make plays in top and bot and help contest objectives. Bruiser/Tank champs (or their counters) will usually stay top because of requiring less presence early game before team fights break out so they can focus on leveling and farming rather than roaming. Teleport is popular on these champs so that they can assist elsewhere without losing much time in lane or can turn a 4 vs 4 into a 4 vs 5 giving a numbers edge for teamfights around objectives.

Thats because their not in bot lane. Honestly as a support main. I'd still would ward for my adc if it was a yi or something. The reason why supports buy all the wars for bot is so adc can rush dmg items, and sup it just to far away to ward for anyone other than bush near drag for mid.