The first time I heard the term was while listening to an old radio broadcast replay of a Brooklyn Dodgers baseball game. The play-by-play announcer Vin Scully (pictured) said, “on a 3-2 curve ball Pee Wee Reese hit a pop fly into short right field - it’s a can of corn.”

The phrase is said to have originated in the nineteenth-century and relates to an old-time grocer’s method of getting canned goods down from a high shelf. Using a stick with a hook on the end, a grocer could tip a can so that it would fall for an easy catch in his apron.
That explains the ‘easy catch’ but where does corn come into the equation?
In the very early days of baseball, the outfield was called the “corn field.” Especially in early amateur baseball the outfield may have been a farm field.