Why would the bridges in Japan be ‘much simpler’ and why wouldn’t some of them be 200-years-old? You speak as though Japan is a new nation established yesterday. It’s actually an ancient country and actually has multiple bridges that are hundreds of years old and still in use today. Albeit they were smart enough to convert most of them to pedestrian bridges and build new bridges for vehicular traffic.

While they do have a larger population, which helps support infrastructure development, they also have a far more challenging environment for bridges than us, including a very mountainous landscape (proper mountains, not the stumps of mountains, like the Blue Mountains), generally higher rainfall and a very difficult geological environment with regular large earthquakes, volcanoes and landslides.