Again, it is not wrong to make the best product. It behooves any manufacturer to sell to distributors with largest reach, especially if it is a non-exclusive agreement, and this is perfectly normal market activity. You seem unaware of the legal definition of monopoly; Valve is nowhere near it. The made up internet definition, having a majority of sales in a market, is just what happens when the product is good. Actually it would be a bit of a market failure for the best product to not have the most sales.

Also please don't point to the failure of Epic or other stores; they're just bad products. Epic store didn't even have a shopping cart for years. No one competent is competing, and that's not Valve's problem.