The answer is DRM.
JAV is distributed using several methods: DVD, BluRay, download, and streaming all protected by some form of DRM. In order to get those movies to people like us for free the DRM must first be dealt with, there are 2 ways: DVR it or remove the DRM from the original file.
The DVR method involves that you screen record the movie as it plays in real-time therefore taking a very long time (the duration of the movie) and encode (lossy) to an output file. The higher quality you output to the more computer resources are needed. But the best thing about this method is that this works with all formats, DVD, BluRay, download, streaming, etcc. DRM or not, as long as it can be displayed on your computer screen it can be recorded.
The other method is to remove the DRM from the original file. This is the ideal way because it produces a lossless output and the process is very fast.
Removing DRM from physical copies (DVDs and BluRays) is easy. The specifications are standardized to be compatible with DVD/BluRay players (read: not proprietary) so once you figure out how to crack one you can crack them all. People figured out how to crack this a long time ago and so if someone has a physical copy, decrypting the movie is quite easy.
But nowadays JAV is primarily distributed through downloads and streaming. In order to crack the DRM on a downloaded file it requires you to connect a server and/or use a proprietary media player so its a bit tougher. But as with all DRM, time is the answer and eventually people figured it out. This is why even though HD has been available for a long time HD rips have only became available relatively recently. Yipman and others operated in this way, decrypting HD video downloads from DMM then distributing them DRM free to the public. But there has been some DRM implementation changes recently (about a month or two ago), putting people like Yipman at full halt until it is cracked once again.
In effect we been thrown back into the stone-age, before DMM’s DRM was cracked, leaving left only the low quality DVR rips and the occasional high-quality DVD/BluRay rips.
Well there you go. Hope that explains it.