Why "Piggy Bank"?

because it looks like a pig, maybe.

Piggy bank (sometimes penny bank or money box) is the traditional name of a coin accumulation and storage container, most often used by children. A piggy bank is known among collectors as a still bank as opposed to the mechanical banks popular in the early 20th century. Piggy banks are often shaped like pigs and made of ceramic or porcelain. Piggy banks are used to reinforce ideas of savings and spending to children; money can be easily inserted, but the pig must be broken open for it to be retrieved, forcing the child to justify his or her decision.