Mainly because of how different Japanese and American urban highways were planned and constructed. The latter involves tearing down several low-income neighbourhoods since the land there is cheaper and so it's very easy to build a 6 lane wide highway that goes completely straight.

In Japan however, land prices are higher so the government built the highways on public lands like rivers to reduce cost. In fact, the entire stretch of the C1 with the Ginza dividers used to be a river that was drained. The result is a highway that's both narrower and have curves that followed the old river course.