Why are bottom ADCs always AD?

No, saying "Coz ADC stands for ATTACK DAMAGE carry and AD is ATTACK DAMAGE u scrub!!!!" does not count as answering the question >:( Why are the bottom carries always AD guys with guns or bows? Is AD inherently better than AP lategame, while AP simply works better at early game, thus making all AP carries go to mid and all AD carries go to bot for the support? I doubt it because Zed exists in mid. Why don't AP champs get supports?
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