Because it’s tradition, behold the description:

*“If all the Ponyarchs where to be gathered in one place, the one you’d spot first would without a doubt be Big Macintosh of the Iron Stallions. Standing significantly taller and thrice as broad as the other Ponyarchs, Big Macintosh sure was an awe-inspiring sight to behold. And equipped with his trusty hammer, McBig, which was equal to a Ponyarch in terms of size, he was a dangerous and horrifying adversary on the field aswell.

A stallion of few words, Big Mac prefered to remain silent and only speak when really necessary, most of the times he would reply with a nod of agreement, or a shake of his head. He was also very leveled-headed and never let others in on his feelings , which led the other Ponyarchs to assume that he didn’t have any, or simply didn’t care. But the truth was the opposite, inside himself he harbored a lot of feelings, most of it being pent up anger and envy. Oh yes, so much envy, he was constantly envious of his fellow Ponyarch Applejack, the oh so popular sister of his. She was chosen to be the architecht of the great fortress of Terra, a task Big Mac thought himself a lot more qualified for. And to make it worse, she liked to constantly gloat and remind him about it, wether she did it as a little tease among siblings or not, he did not care for.

With no one to talk about his problems with, the Ponyarch of the Iron Stallions prefered to vent all his anger and rage on any and all who had the unfortune of corssing his path during his crusade throughout the stars.”*

Also, he likes dolls. And he hates it when people mess with his collection. So unless you want to be used as a human(pony?) golf ball, you’d best keep away from Smartipanticus.