Kansai is a great airport. Flew there many times in the 747, back in the day.
A huge engineering marvel. The entire island was artificially created, built by piling up material in the bay. The terminal itself has hundreds of jacks underneath to account for the planned settling of the material over time.
At night, it looks just a bit like an aircraft carrier - a lone runway in the middle of dark water. Much longer of course.
And the crosswind there can be fierce. Wind rips across the water from Kobe, with nothing to stop it.
The downtown airport was both land-locked (buildings etc. crowd it and limit its length) and surrounded by terrain. Longer runways required for international flights could only be built outside the city. This was the solution and it's very cool.
The Hong Kong airport Chep Lap Kok (which replaced Kai Tek) was done after Kansai, and borrowed some of the reclamation idea. They dug out a mountainside for the material and built an artificial island next to that mountain. Same reason - longer/more runways needed and no room downtown.