Except economy of scale should have lowered that cost dramatically. The Steam of today is incredibly profitable compared to the Steam of 1994, that means costs have gone down, but their commissions haven't followed, and this is why others want their own store. If their commissions were fair, it wouldn't make business sense to compete with them.
Planting and processing my own wheat for bread would be very resource-intensive, that doesn't mean commercial bread is a luxury article.
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.
The combined reputation of all four wouldn’t reach Valve levels. If not net negative. Difference is too large between Steam and any of its high-profile competitors.
Planting and processing my own wheat for bread would be very resource-intensive, that doesn't mean commercial bread is a luxury article.