>Even if that’s true (and it gets repeated as a very general statement without general data to back it up, so I’m not sure), then why couldn’t payment processors either a) charge retailers in high-risk segments a higher transaction fee, b) charge merchants penalty fees if the number of chargebacks from their transactions exceeds some threshold, or c) compete with each other to serve higher-risk markets?

They do all of the above. There are obviously an endless number of niche payment providers who serve the segments. And they are... expensive and unreliable. Not suitable to a huge business like Valve that derives the majority of their revenue from mainstream products.