So that customers who don't want to have to mess with emulators can just buy a game and play it?
That's one of the primary reasons to use GOG - even the ancient games they sell (in my experience) Just Work. They would be far, far less successful if every customer had to become an expert in DOSBOX in order to play an impulse buy of a game from their youth.
I do not understand why they keep (kept?) bundling the games' datafiles with the emulators/runtimes.