A person 亻 beside an ox 牛: the act of butchering divided one animal into countable pieces. 件 became the word for 'a piece of' anything separable — clothes, luggage, documents, events. The ox disappeared; the counting stayed.
件 is the measure word of adult life: 一件衣服 a piece of clothing, 一件事 a matter, 一件快递 a package. E-commerce reports speak of 亿件包裹 — hundreds of millions of 件 moving through couriers every day.
A person 亻 splitting an ox 牛 into pieces — each piece is one ITEM.
邮件 'mail-item' became email's Chinese name — every message you send is technically one butchered-ox unit of correspondence.