All eight American Idols from the previous seasons performed a tribute to Simon Cowell last night -- but David Cook was noticeably absent from the stellar lineup ... and now we know why.
A rep for David tells Access Hollywood the Season 7 winner was at a charity event in Kansas City.
Despite missing out, Cook twittered "Congrats to Lee DeWyze! So sorry I couldn't be there to see all my fellow idols. I'm there in spirit!"
