A few of these games have been re-released later, and in some of those cases old engineers had to be tracked down who still had the source code laying around in their personal archives.
I imagine this is very common, considering some of the titles I've witnessed this happen to. If games will those types of budgets and followings were lost, I can't imagine the vast long-tail of the industry being any better. Plus there were far more independent studios at the time who later "made it big" only to flame out. I expect the vast majority of content created during this timeframe is permanently lost.
These days with the advance of business types understanding the long-term value of IP, I expect this is much better controlled by whomever is underwriting the cost of development. I believe this likely to be a bad thing - as you'll see more lawsuits over old games in the future.