The Japanese started sleeping on futons when they evolved after the Japanese slept on mats that they would roll out.
The Futon supposedly is more comfortable and better for the Japanese peoples backs and posture however for me I find that Futons are uncomfortable and sleeping directly on the floor is much better.
But I also prefer my mattress.
Not all Japanese sleep on futons as some Japanese still sleep on mats or cushions laid out on the floor.
The Japanese also traditionally sit on the floor when eating at a table.
When the Japanese sleep on the floor the Japanese tradition is to sleep on top of a precisely arranged combination of cushions and mats.
Then at the bottom is a tatami mat, followed by a Shikifuton (or mattress) and a kakebuton (the duvet), and topped off with a buckwheat hull pillow.
So yes the Japanese do sleep on the floor but they also use cushions or a small mattress on the floor to make the sleeping on the floor more comfortable.
I sometimes will lay a blanket on the floor along with a pillow and will lay there watching TV and will easily fall asleep.
Sometimes I get some of my best sleep while sleeping on the floor.
Some Chinese as well as Mexicans sleep on the floor along with the USA.
But not everyone in those countries sleep on the floor but it's up to the person themselves if they are comfortable sleeping on the floor.
I have a Japanese friend who I visited about 10 years ago and she slept on a bed and not the floor.
Also some Koreans sleep on the floor but not all.