Steam tax

I just bought Dawn of War 2, the sequel to the massive juggernaut of tactical gaming Dawn of War from Relic Entertainment, so I thought I'd offer some thoughts on it. But first, let's gripe about digital distribution. Why on earth am I paying 10$ extra to buy the game on steam? Look, publishers, I LOVE game boxes and special editions and such. But I hate optical media discs. The reason being I always lose them and never, ever, use my optical drives anymore so I'm never sure they're installed. Steam on the other hand is always ready to go. I will therefore always buy games from Steam, but if I like the game I will probably buy a special edition box, for art books and such. I'm certainly not alone in this since Steam has, at least, over 15 million active users, so why on earth are you charging us a "steam tax" when your obviously already making more money from a digital distribution sale because you won't have to pay for box printing and such?

I suspect that the answer is either "publishers are dumb old-school people with no clue" or possibly "publishers want to make third party digital distribution unpopular and then launch their own digital distribution platform when it's ready in 2025". Because these are the only answers that seem to cover all the information. Neither is a very good answer, anyone have any alternative possibilities?