Because we started with glow engines that NEED to be broken in....Old habits are hard to break..

Direct quote from the Zenoah owner's manual.."No specific break in is required..The engine is gradually broken-in as it is used and the output is also gradually increased"
The engineers know more about their product than a bunch of old fogies (myself included) on an engine forum...
More from the manual..
Oil mixing ratio is 25-40 to 1
The mixing ratio is according to the OIL RECOMMENDATION..(capital letters mine, for emphasis)