companies have tried. The most successful when it comes to user loadings is SOG, but the amount of fiddling around you need to do gets expensive. I had a perfect A/V theater and stage tech load in my SOG (Including robertson driver for extension cord plugs, and a long fine pot switch screwdriver) But that took a lot of work. And the SOG is constructed to a looser tolerance to make that work.
I think the big companies just don't think enough people want to do it. That said, Leatherman is making it easier to take apart the larger tools than it did before, so who knows, maybe they are going in that direction?