Note that stick built most of the wood is cut to size at a factory. If the wall is 8 foot, then you just grad the precut stack of 92-5/8" studs (after adding the top and bottom plates that works out to 8 foot with some needed slop for the ceiling and flooring), likewise you will get 104-5/8" for 9 foot walls. No other wall size is common in the US because then you have to cut all the boards on site for higher costs. Likewise the windows and doors are standard sizes from a factory. No plumber is making pipes on site, they just use a factory made pipe, valve, sink... A modern house really is a kit, there are just enough different size options and styles that there are many factory options and it seems each is custom even though most of the parts are standard.
Yeah, when people think "modular home" they think pre-assembled wall panels, etc, but those have other problems (like it's hard to wrap the insulation around the walls when you have to connect them).
Real expensive custom homes are where you change dimensions so that non-standard parts are needed (not even things like wider doors, but non-standard widenesses).
