I know this answer will horrify you, but in Japan, a lot of them DON'T.
Many, many great games have been lost over the years. Some are put on tape, which goes bad, others are in closets and the people who were supposed to be keeping track of them have left the company.
The thing is, a lot of them use obscure data formats -- have you tried importing a 15-year-old Alias|Wavefront model into Maya lately?
In many cases, you could simply not recompile some games. For example, at my last company, our PS1 game would only compile on ONE hacked-to-shit machine running Windows 98 -- the environment breaks under XP or 2000 -- that no longer exists (thrown out, I believe). If you told me to make a new build of the game, I'd laugh at you!