If we're going to do a one sentence summary here, it's "Japanese men don't enter the dating world without a job, and if they don't get that after college it won't ever happen".
That's only one of the subcultures who can't get a date. I was rather talking about who do have a steady job, i.e. are "Salariman", but in nerdy careers, i.e. tech, accounting, back office, etc. and have given up, or never tried, to be physically attractive to potential mates. The types who have lots of disposable income and would roam the streets of Akihabara to buy manga and go to maid cafés, or drop by Yodobashi to get a Seiko TV watch.
That's not a small subculture in Japan, and Japanese women are absolutely repulsed by them.