I think the OP is missing a large factor in the cost of manufacturing, which is expertise. The reason a super complex Tacoma is $40K and the relatively simple Fleet + flatbed that goes on it is also $40K is that Toyota spent $1B on designing and building a production line with all the tooling, robots, jigs, fixtures, training and what not to allow them to build a Tacoma in 8 hours. Furthermore, Toyota has been doing this for decades and has spent billions in R&D to obtain the expertise necessary to build a production line to build a truck so efficiently, so they can sell it so cheaply. In order to be profitable, they need to sell 500,000 Tacomas to recoup the cost of the production lines, and to fund the R&D to make the next gen of Tacomas.
FWC doesn't have the capital or expertise to do this, so their campers are built very inefficiently by hand with semi-skilled labor. However, FWC still has expertise, they know how to put together their campers so they work fairly well and last a long time. They know what thickness tubing to use and the length of every tube, they know where the welds need to be, they know the geometry of the lift panels that works, they know exactly what wires go in each harness, and how long each wire needs to be. When they have built one, it is ready to go.
I DIYed my previous camper, which was a pop-top UZJ100. It took me way longer than expected, cost me more than expected, and then wasn't as nice as my current FWC. I had never built a pop-top land cruiser before, so I spent about 6 months of lunch breaks and evenings making sketches, researching components, finding suppliers, making schematics and CAD models to make sure things would fit. Once that was done, I spent another 3 months of evenings and weekends actually building the thing - making the new top, cutting out the roof, sewing the soft walls, making cabinets, running wires and hoses, painting and sanding. I then started using it and quickly realized as I had never built one of these before, many of my great designs didn't work as well as they should be and needed to be revised. So I spent a weekend or two a month for the next year re-doing all the things that could be improved. Now I (mostly) had fun doing much of this, so I didn't mind the time sunk, but even if paid myself half my salary rate, there must have been $25K of labor in that thing. If I were to build a second copy it would have been $10K, but I don't need two and it wouldn't be nearly as much fun.
Most folks who talk about how quick and easy it would be to build one of these, only think of the middle part - the time/cost to actually put it together. They ignore the countless hours figuring out what you actually want to build, and how you would actually build it, then the iterative process of re-engineering/fixing what you did build so it actually works the way you want. If you want to DIY, go for it, but realize it will be for the fun of it and really won't save you any money.