The classic games are very old. They were written for OSs, drivers, APIs, etc that are now depreciated.
As such, Bliz is selling the games “as is”. Meaning, they acknowledge that they will have compatibilty problems, which will get worse and worse as time and technology progresses. They task the community to resolve issues that arise from age.
The only exception I see is the issue with Diablo 1’s Battlenet gateway connectivity. That appears to be a bug and Bliz is aware of it.