Japan's economy rose during the last half of the 20th century with the help of contract workers who came to the big city of Tokyo from different parts of the country.
They helped construct part of the present metropolis of Tokyo and now they are, like the rest of the economic boom, an echo of the past. 

There's no more work, and the economy and/or social system can't or won't sustain its human waste. Maybe it's symbolic of a consumer economy on its return from a long economic crisis that the country just about is coming out of.  

Who these men are and why they never returned home is a question that was probably never asked. In the new consumer-centric Japan it's just a matter of use and throw out. 

Sanya is one of the neighbourhoods where all these men still living and surviving.