The nice thing about a factory built house compared to an independent contractor is you don't have to wait weeks between job types, your plumbers are on hand as soon as it's ready for them to get started.

With that said, the advantage disappears compared to national builders- the guys who buy up big farm fields and build entire subdivisions all in one go before they even have buyers. They can keep crews rotated between jobs in a fairly predictable schedule, so the only thing holding anyone back compared to the factory is bad weather preventing digging out room for foundations.

Even you little homebuilder can do that. the planning is standard and predictable so they line it up as needed.

you as a one off can't because the plumber isn't going to give you the priority needed.