Because it's dressed as lamb. I sometimes get a 'leg of lamb' from NZ and it came from a damn big lamb.
"n the early 1900s, mutton was widely consumed in the United States, but mutton consumption has declined since World War II.[12] As of 2010, most sheep meat in the United States comes from animals in between 12 and 14 months old,[13] and is called "lamb"; the term "hogget" is not used.[14] Federal statutes and regulations dealing with food labeling in the United States permit all sheep products to be marketed as "lamb."