It's not really a thing in the global north, but in many places in the global south you'll find street vendors selling soup and other whole ass meals.
First reason that comes to mind I think is how many (official or ersatz) public spaces to sit down and eat (you can't really eat soup on the go) exist, and also how soup-centered the local food ways are.
I'll go to my university's library to see if they have anything on the anthropology of street food, and report back.

soup is supposed to make you feel like a big watery barrel. full of soup
dont worry abt it eat some more soup

Average store after 7bicyles takes power