Quake and Half-Life are not really retro yet, even though they are fairly old. I'd guess the main reason for this is that people can still run it on their new systems without having to worry about any of the weirdness in downloading an "emulator".
Take a look at everything software-wise that is considered retro. It's mainly retro because it runs on yesteryear's hardware. When people mention it, the average person might think, "oh yeah, I remember that.....back in the day...". However, they've stopped using it because they've moved onto newer technology.
Will Quake and Half-Life ever become retro? Sure they will. I would almost go as far to say Quake is retro now, but Half-Life might need some more time.