A big reason it feels like nobody competes with steam is that if you want to sell your game on steam it can't be cheaper elsewhere. So any other store can't compete on price.

If you don't sell your game on steam you are missing 90% of the market. So as long as Valve continue to make steam good enough, nobody has an incentive to switch.

It's an abusive monopoly. Steam take 30% of revenue from developers and Epic take 12%, but the prices can't be 18% cheaper for the consumer without giving up 90% of the market!