There was a New England company that sold complete kits for small houses with pier and beam foundations, designed to be constructed by two fit adults, including the fasteners! I think they went out of business in the 2000's.
EDIT: I just found another company doing almost the same thing: https://www.shelterkit.com/gallery.html
I can't speak to other kits -- some of the shipping container ones actually seem like they might work for some people (https://simpleterra.com/) but we looked into working with Amazon kits and it's really tough. The two main issues are on-site work and bringing the kit up to a livable standard.
On-site work is expensive -- think $50-80k depending on the site to secure a building permit, level the ground, and pour a foundation. There are some new types of low impact foundations (closer to $15k + $20-30k for permits) we've seen and are exploring but I don't know we could do that with a kit.
The main issue is with turning the kit into something livable. Kits are usually difficult (and loud) to assemble, the wood needs to be finished so it doesn't rot, they usually aren't designed to accommodate plumbing, gas, and electrical lines, and unless you want to spend hundreds of hours of your own time to learn those skills, you'll need to manage and hire 5-10 professionals which quickly eliminates the initial cost savings.
These kit homes definitely exist -- the big problem is that the site work is often 50%+ of the actual costs of installing the unit. We definitely intend on bringing down site costs as we make our units lower impact on the property on which we build.
What's the best kit home in the U.S.?
It would be neat if there was a (high quality) $50k kit homeowners could buy and assemble themselves, almost like IKEA furniture.