The 6-pack was an actual retail release at one point, before 7 and 8 were made I think. GOG simply got the rights to sell that release in digital form. There is often a lot of unexpected legal stuff to sort through when releasing games digitally, as any release that can officially be considered "separate" from earlier releases (e.g. selling only M1-5 as a pack, or packing 6-8 together) requires creating a whole new set of paperwork and whatnot.

Indeed, I own that release on CD. Fair enough.