I would think that many old Korean movies actually *are* out there and available in Korea and perhaps other Asian countries. The demand for older Korean movies is not as high as newer ones, though, for most non Korean fans, so most would likely not be subtitled or distributed outside of Asia. Plus, with titles that never made it to DVD, they would be almost impossible to find online anywhere, even Korea. This goes for any country. Have you ever tried finding old Taiwanese movies subbed and online? Unless they were made during the kung fu craze of the '60's and '70's, it is nigh impossible!
The demand for older Japanese movies is much higher. There is a larger market even outside of Asia for them, so that is why you can find them much easier and many with subs.