A chaat vendor cooks almost nothing at the moment of sale. The plate is assembled from prepared components in seconds, and that workflow is dictated by what makes chaat work.
The dish is defined by contrast that decays
Chaat depends on crisp elements sitting against wet ones: fried dough or puffed rice against chutney, yogurt and softened potato. The contrast is the dish.
Once the wet components touch the crisp ones, moisture begins migrating into the fried element and softening it. The window in which the plate is at its best is measured in a minute or two.
Assembling to order is therefore not a convenience but a requirement. Anything premixed arrives uniformly soggy and is a different and worse food.
The components have different useful lifetimes
Fried elements keep for days if they stay dry and sealed, since their low moisture content is what preserves them. They are made in batches well ahead of service.
Chutneys and boiled potatoes are prepared daily and hold for a service. Yogurt is whisked as needed. Each component is produced on the schedule its own stability allows.
Assembly speed is the vendor's actual skill
A chaat cart handles a queue by having every component within arm's reach in a fixed layout, so building a plate is a sequence of practiced motions rather than decisions.
Because nothing waits on a stove, throughput is limited only by hands. One person can serve a crowd at a rate no cook-to-order stall could match.
The fixed layout also allows variation, since the same components in different proportions produce different named plates from one station.
Seasoning is applied at the end for the same reason
Chaat masala and other dry seasonings go on last, on top, where they meet the palate directly rather than dissolving into the mixture.
Their aromatic compounds are volatile and would fade if mixed in earlier and left standing. Applying them at the moment of service preserves their impact.
Why it is hard to package
Retail and delivery versions of chaat generally separate the components and ask the customer to combine them, because no packaging prevents moisture migration between them.
American Indian restaurants face the same constraint, which is why chaat appears as a plated appetizer eaten immediately rather than as a takeout item. The dish resists every step that would let it travel.