Note: The phrase origins of many common idioms are unknown. Thus, when that happens, I'll usually just list some of the popular or plausible theories that exist for how a phrase may have originated. Or, if not that, then I'll try to find the earliest known written quote of the saying.
Quotes will often be taken from old books, newspapers, poems, plays. These are to give an idea on how far back in history the particular expression goes. However, do not assume that if I, for example, quote a newspaper from 1852, that the idiom originates from that time. Indeed, since the expression is already being used by the newspaper, then it's probably already a well established expression, and is thus older.
KnowYourPhrase.com - The Meanings and Origins of Phrases
This phrase may come from the pleasantness and ease when it comes to eating a pie. Another expression similar to this one is 'a piece of cake,' both expressions share the same meaning.
During the 19th century, the word 'pie' was used to describe someone as being delightful or to depict something as being easy. For example, in the novel The Adventures of Huckleberry Finn by Mark Twain, 1885, the word 'pie' was used as a nice remark:
"You're always as polite as pie to them."
Two years later, the phrase is seen in various newspapers. For instance, in The Newport Mercury newspaper, 1887, there's a part that reads:
"You see veuever I goes I takes away mit me a silverspoon or a knife or somethings, an' I gets
two or three dollars for them. It's easy as pie. Vy don't you try it?"
Thus this idiom goes back to at least the late 19th century.
* Instead of cooking myself a burrito for lunch, I'll just have a bowl of cereal since it's easy as pie to make.
* Janice was once afraid of water because of not knowing how to swim, but after a few swimming lessons, it's now as easy as pie for her to get into the water and swim.