Why are manufactured homes so much cheaper?

Because the land is the most expensive part. Especially if it's been improved like sewer and electric.

the land is the expensive part. houses need permits and have a lot of specific regulations they need to follow. trailer homes not so much, also cheaper materials

I've lived in 4, they are cheap because its one floor and mass marketed. As someone has said the land makes it more expensive.

They're built assembly line style, improving construction efficiency and reducing costs.