There are some people that do because it's easier to acquire the game. I know this is a niche of a niche but this Avangate reseller has limited number of payment options. Gog / steam are modern distribution platforms with modern payment options.
Not everyone pays with credit card via internet and gog / steam allows to pay without them.
There are plenty of people who would gladly pay and buy this game, but this Avangate only allows credit card.

Also there is other factor that your download links will not dissapear in case you will loose your dvd.
If this game had more payment options available, i'm sure more people would buy it.

It's just about making life easier for everybody, everyone wins, devs because more people buy the game and people because it's easier to get it.