Skill shots getting shafted

Skill shots are difficult to land, most are easily blocked by minions, they usually cost a good amount of mana, some are slow moving. Yet for some reason, some champions, including level 1 ADC, are doing more damage with a single auto attack than a mage can do with a skill shot. Explain the logic behind an auto attack from an ADC hitting harder than a skill shot? Explain to me why Vel'Koz has to perfectly aim, and perfectly time his skill shot to do 60 damage while depleting his mana, while Vayne or Lucian just needs to tumble towards you and do 100 which can't miss and barely uses any mana if any.