Why does Zakat not have pillars like Salah, Sawm, and Hajj, but only conditions for validity?

There is no difference between a condition (shart (definition)) and a pillar (rukn) except that a condition is external to the essence of a thing, like ablution (wudu') for prayer, while a pillar is internal to it, like al-Fatihah for prayer. Jurists have expanded the usage, calling a pillar a condition, because both are indispensable for the validity of an act. Some of them have explicitly mentioned the pillars of zakat (definition).