Painting actually introduces some scaling issues in manufacturing. Any painting step prevents you from haphazardly mixing all the parts produced on a line. Most vehicles will have multiple lines that eventually converge across multiple suppliers. Painting forces you to either coordinate the production of each individual component so that they're all painted the correct color during final assembly (which becomes difficult if any parts fail QA), or buffer painted parts in a giant warehouse. There's other advantages to the warehouse, but I wouldn't call it cheap.

Very common to use a mix of both strategies in practice.