Historical reasons, If you have ever tried mounting a really big camera , like an 8x10 Deardorff, on a tripod you'll understand.
Unfortunately I very briefly experimented with 4x5" (unfortunately because now I really miss the 4x5 aspect ratio (I don't like cropping) and the movements, but it was just impossibly slow to shoot (I was slow) and to bulky to carry around and expensive) with that camera I would have loved 5/8" better yet two or three 5/8" screws. so yes I know what you mean but I still think 3/8" is not overkill since it is just a very short piece of thread that is actually used.
Regarding studs, any decent hardware store (what do you call them in your country?) will have studs in various lengths , diameters and thread counts.
well the name for hardware store would be "Baumarkt" (Bau=Construction) but in switzerland we don't use that word. we mostly just call the store by its name (name like bhphotovideo) or try to explain what we mean (we do that a lot in switzerland) like "I have to go to that store where I can buy screws and tools and such"
but did I ask about studs? if I did, I think I didn't want to
but thanks anyway.