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 mostly water. Water is big and heavy. It can spill. Burns can be devastating. If you have a big pot you need a very flat stable surface to heat it.

Sausages can be cooked as you need them but you can't keep topping up your soup all day. You need to make the correct quantity to begin with. Which means waste is likely.

It can be done and is done in Finland, or at least Helsinki -- there are a bunch of stands where you can get a salmon soup by the docks. I think a would-be soup vendor in, say, burgerland, is up against cultural expectations about soup, namely that it comes in cans and sucks and you only eat it when you're sick.

Salmon soup (lohikeitto) is common to find in most Finnish markets and restaurants. Usually exactly as you described, large vats of soup and bowls that are quickly served to each person.

Another perfect food on a cold Finnish winter day is rice pudding (riisipuuro). It has all the advantages of soup but it’s also sticky, so no concern of clumsily spilling soup while wearing thick gloves/mittens.

Ramen, pho, and other soup styles are also common streetfoods (not in Finland) for the benefits you mention

You should be able to check out soup at public libraries.

Canned soup makes people think soup is mid. It's tragic. Take the bones of a smoked chicken and make a soup and that's not just lunch, it's a whole dinner

The problem probably lies with the fact that if there's a soup and a hotdog or burger side by side people don't buy the soup. Or that they're willing to walk 200meters to get the hotdog/burger instead of the soup.

The vendors are just trying to maximise and getting whatever the audience is more likely to buy.

everyone in here is wrong

a decent waterproof disposable bowl plus a spoon is much more expensive than the wrappings you give finger foods. and they gotta be nice, i don't care how nice your soup is, if the container i got it in starts leaking or i get burnt i'm not having a good time

There's soup in every corner store deli in the NE US

It's because you can't huck fresh, piping hot soup at people's doorstep in the morning.

that's what I'm saying!

Average store after 7bicyles takes power

Counterpoint: soup sucks

In my experience, most soup places kinda suck. It's not even a dunk on soup, just companies that specialize in it and aren't phó/ramen tend to blow.

Chili tends to make a splash, but likely cause it's better year-round and (being a thicker product) doesn't slosh and splash as much, making it more portable. To add one more factor to consider, is the spoons and cups, making it more of a hassle for both stocking in a food cart as well as easy consumption while on one's feet.