What you say is true if "online game stores" were fungible commodities. The reality is that Steam has a long-running reputation of generally (but not entirely) pretty good for gamers. Buying games on Amazon, Epic, EA, Microsoft, or most any other storefront is risky because they don't have ~20 years of good pricing and good-enough governance. The only storefront that meets or exceeds Steam's reputation is GOG, mainly because they sell DRM-free games that work with or without GOG's continued existence.
Maybe if the other big-monied game stores were to have a similar show of goodwill towards players would Steam feel some real competition.