AP x AD on mid

Could you explain me. What advantage has AP champ (mage) vs AD champ? Because If I play AP champ I always get such a incredible situation in late game. For example I play Syndra mid against Yasuo. Everything is Fine im fed BUT BUT in a late game he can still kill me and doesn't matter If I have 7 or 15 kills. So Where WHERE is dissadvantage the AD? It has lifesteal crits and it deal XXXX dmg on 1 basic. Why WHY I cant do dmg on basic as a mage and If I have all in CD and He too He still can kill me but Im not so what's the stupid thing about that. I rly want to know what is advantage of mages or they are useless and we may play all AD champs only? What do you thing? Thank you for your answers.
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