perhaps the question is the wrong way round ...... why not domed heads on oriental and American bikes?

They used domed because that is what should be used if you have a bit of pride in the way you design bikes, and how they look. A pointless expense you say? well that depends on your perspective.
They stopped using them when they had to compete with the souless bikes built to appease the gods of the balance sheet, where every cent has to be shaved, every dollar grabbed.

When rebuilding my bikes I also domed the heads of the stainless bolts using a rednecks lathe, i.e. a cordless drill and an angle grinder. Quick, simple though perhaps a tad hazardous.