Completely agreed. But even if the will was there, how do we know the source code even exists? 1999 was a long time ago. Source code is company IP. Generally it's the devs who write it and care enough to want to release it to the masses or archive it properly. But they are not the decision makers. There are too many stories of source code quite literally disappearing despite the devs best efforts. A lot of the times when we do get access to source code it comes from rogue behavior aka a former dev putting their ass on the line for the community (see the recent MGS leak on 4chan).
Also like a sibling comment has stated there is no guarantee that the code would be a in a usable format. With the crazy crunch culture in game dev, I doubt most of these studios from the 90s were pumping out clean code. Like an archaeology dig, you'd have to budget a non-trivial amount of time getting what you find in a state that is useable by modern standards.
All to say, I think reversing these older games is a worthy endeavor that is not going away anytime soon. I for one, am excited by the productivity benefits that AI tools have brought with them.