One could argue Steam is a case of monopoly that is mostly "caused" because the users keep choosing it even though there are plenty of options to buy from. But, as you pointed out, most of those options don't offer the quality of service Steam does, even the ones that can financially invest to bring them on par, it really feels like they don't want to. Like Ubisoft store is shit, but people have to download it because their game only works through it even when you buy from Steam