They do handle all hosting, distribution anything related to game's actual sale.Damn that's a high cut
There is also the fact that they have the carefully polished feature set and UI thats been devved over the years.
Steam could definitely lower the cut and still be massively in the green, dont get me wrong, but something like even 20% would be equitable trade when you think all you get by being on Steam.
Also, I too, have just bought it.
And saw two of my Steam friends also did, so yay.