Where do you get that "government has the programs to help jobless/unemployed people" impression? Japan social programs are very conservative by Western standards. A young person who don't/can't get with the program (either get educated and find a good job or accept a low-requirement job that means long hard hours) won't get any easy or generous benefits. Japanese people are well-off in general because their economy is strong since WWII, but lots of people (very randomly let's say 1% of the population, that's still a lot of people) still can't get with the "program" so there are plenty of "supply" of fresh meat.
I think every answer so far still miss part of the original point... Hard work vs. Easy money is true EVERYWHERE. There are many countries that seems more likely to offer up endless willing girls to make porn. Japan seems unlikely candidate for top 3 porn producer of the world.
My pet theory is:
(1) Japanese tradition (pre-WWII) view sex (and female virginity) very differently from either Christians or Confucian cultures. For the average folks, marrying a non-virgin (say a widow) is no big deal. Even for elite society, officials, rich business men and samurais, marrying a "professional girl" (geisha) can be a social status badge of honor if she has valued qualities (mainly culture and education) and her sexual history can be totally overlooked. That is quite unique. A girl with a history of prostitution doesn't ruin her social status. Another twist is this: many many girls/women had experienced sexual violation or even r*** but it doesn't affect her life much (e.g. finding a good husband). Pretty much no one expect a girl to have one and only one sexual partner in her lifetime (according to expectation of both Christianity and Confucius).
(2) I could just stop there because everything else is a derivable result from that. An ex JAV performer or hostess etc doesn't become human trash and social garbage. She can find a decent husband (not saying every Japanese man is willing to marry a former sex worker, but there are enough of them around). Some of them even can upgrade her celeb status to get gigs on TV (game/variety show etc), which is mostly unavailable to US porn stars (there are high profile, but extremely rare exceptions). Even if both options failed she can still work as hostess/prostitute until old age, which is (almost unique to Japan) not a terrible lifestyle.
For most of the world, even a short career (1 vid) in porn would pretty much ruin a girl's life and close many many doors. Once you take that away, and given Japan economy/society is male-centric, meaning a woman's life expectation is to be some insensitive dude's wife, really his maid, or working for some male boss jerk who give you all the work load but no chance of promotion.
