Older games are often already flaky on the current versions of the platforms they were first made for, there's no great easy business in taking legacy PC games and fixing them to be well emulated/translated.
Well if you go far back enough (like games that need DirectX8 or games that don't work on W10), the compatibility layers get really good, and overhead doesn't matter at all because they run at a billion fps.
Except for MS' Direct Draw implementation under Windows 8 and beyond, were your 1999 game runs like a dog under an i5, and you need a wrapper to translate DDraw to OpenGL.
