How though, are pies thought to be easy? They aren't especially easy to make; I know, I've tried it. The easiness comes with the eating. At least, that was the view in 19th century America, where this phrase was coined. There are various mid 19th century US citations that, whilst not using 'as easy as pie' verbatim, do point to 'pie' being used to denote pleasantry and ease. 'Pie' in this sense is archetypal American, as American as apple pie in fact. The usage first comes in the phrase 'as nice as pie', as found here in Which: Right or Left? in 1855:

"For nearly a week afterwards, the domestics observed significantly to each other, that Miss Isabella was as 'nice as pie!'"

From: http://www.phrases.org.uk/