Why can't the studies simply release an official "no-CD" patch to solve this problem? If the really don't have the source code anybody, just ask politely at the scene. ;-)
GoG sometimes does that for retro titles.
gog sometimes ships with official scene no-cd release exe's
If it's 'official', it needs to be tested and supported. Why throw money at a years-expired project for a handful of users who won't pay you?
Because if your game does not work on modern OSs because of the DRM that you used can easily become a marketing disaster.
How many studios or publishers are still in business under the same name? And how many gamers would actually care? Even in the worst days of Ubisoft's "add DRM until it doesn't work for 50% of legitimate users" approach people still bought their games.
There are many older games that still have an avid fanbase (which can imply potential for a shitstorm).
StarCraft is the exception that proves the rule, and Blizzard did actually release an official no-CD patch. Other than that, what games, and what company would they impact? E.g. my friends still play Total Annihilation, but the company that published it went bankrupt years ago; Gas Powered Games might theoretically be affected by their reputation but they probably don't have the rights to release a patch for Total Annihilation even if they wanted to.