Something that is “easy as pie” is something that is very simple and easy. “Easy as pie” has been cited in print since at least 1879. “Easy as eating pie”—perhaps the meaning of the saying—has been cited in print since at least 1888.
 
A similar saying is “easy peasy lemon squeezy.”
 
 
(Oxford English Dictionary)
easy (also simple, etc.) as pie.
1913 Saturday Evening Post 22 Feb. 14/2, I told him what I was up against. ‘All right’, says he, taking it easy as pie, all as a matter of course.
1925 P. G. WODEHOUSE Sam the Sudden xix. 156 ‘How do you propose to make your entry?’..‘Easy as pie. Odd-job man… They always want odd-job men.’
1937 D. L. SAYERS Busman’s Honeymoon xii. 210 He’s knocked out… Simple as pie. No cutting or stealing keys or hiding blunt instruments or telling lies.
1959 C. BUSH Case of Careless Thief vii. 87 It’s in the bag… Everything we wanted and easy as pie.
1991 S. WINCHESTER Pacific (1992) 412 Get up onto the Harbor freeway and then make a left on the Hollywood freeway—easy as pie.
 
1 December 1879, New Haven (CT) Register, “The Corset Conspiracy Case,” pg. 4, col. 2:
“Just as easy as pie; anybody could see that with half an eye.”
 
29 June 1882, The Plain Dealer (Cleveland, OH), “Fergy’s Fancies,” pg. 5, col. 1:
“Suppose the English want to come over from Canada and invade Cleveland. All you have to do it to let her fly, sink the men-of-war, drown the men, kil ‘em all off. That’s all. Easy as pie. Don’t it strike you tha way?”
 
18 January 1888, Daily Inter Ocean (Chicago, IL), “TheSocial Record,” pg. 3, col. 4:
Presently they gave up the pedestrian business for a waltz, and after that it was as easy as eating pie.
 
11 January 1890, Daily Inter Ocean (Chicago, IL), pg. 7, col. 3:
AS EASY AS EATING PIE.
 
Google Books
Eunice and Cricket
By Elizabeth Weston Timlow and Harriet Roosevelt Richards
Published by Estes and Lauriat
1897
Pg. 196:
“Yes, I learned last fall on some of the girls’ wheels at school. It’s just as easy as pie.”
 
Google Books
November 1898, British Chess Magazine, pg. 485:
The key-move is “as easy as pie,” and the after-play presents no feature of novelty.