Older forms of 坐 are commonly interpreted as two people, written 人 and 人, settled on the ground sign 土. The scene directly suggests sitting together. The modern character has tightened those figures into a balanced shape above 土, but you can still find both people leaning toward the centre.
A piece of culture
Please, have a seat 请坐 / 請坐 is a warm, useful invitation: “please sit” or “have a seat”. In Mandarin it is qǐng zuò; in Cantonese, ceng2 co5. 坐 also extends naturally to transport: 坐车 / 坐車 means to ride in or take a vehicle, so the same character works at home and on a journey.