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

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.

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.