That's a good point actually and I agree if it is possible. Some of the gear that drops is identical to your default gear but you have to pray to RNGesus for it to drop. I think the problem with your suggestion might be that the current customisation system is setup with the function of swapping visual appearance by destroying the item you are borrowing the appearance from. And because you cannot destroy your default gear there is no way to swap visuals with it.
It might not be worth the hassle to the devs to code in a different type of system seeing as how you can find gear that looks like your default gear.
Some of the gear you get is stock appearance. For Zerker it is Dynathor, I'm sure I spelled that wrong, weapon parts and G-something armor parts. You can use those or appearance swap with them if you like the default look. I did that for my Zerk since I like the minimal armor look, but I use the cleaner looking axe parts.