No offense, but that's a kind of dumb assumption. They explicitly state that they make the games compatible with modern systems. With a large portion of their catalog being 16-bit, and 64-bit OSes not able to load 16-bit apps, they *need* to be wrapping the games in emulators or the like.
Yes, the original game files - or very close, minimally-patched versions - are in there. However, the vast majority of their customer base wouldn't be able to do anything with those game files. Even if they were, it wouldn't be the simple and user-friendly experience that it is today.
Now, as a Linux user trying to run Windows software, you're pretty much writing off "simple and user-friendly" from the get-go (I've been gaming in Wine since 2006; it's better than it was but it's got a long way to go and the goalposts keep moving). Given that, maybe it would have been nice for the small portion of users who care if they'd provided a "here's the files and instructions you need for setting this up in ScummVM on the platform of your choice" option... but that's not their target market, and frankly it might actually be harmful to their goals (never underestimate the cost of support calls from idiots who think they know what they're doing).
Hence, while many of their games have been *able* to run on Linux since GOG released them, the really core portion of the service - the dead-simple installers and updates - didn't. That is what they're now changing.