While Steam did have a rocky launch, the competitor stores since launched are laughably bad. Seemingly no attention to detail in making a product people would want to use. Epic has a huge war chest from its successes, but the application is still not at feature parity with Steam.

Is there a reason why game developers can't just sell a key on their website using Shopify/WooCommerce/etc. and distribute the game files publicly via a CDN?

Assuming they can get their game discovered via some sort of non-store channel (e.g. Reddit, social media, YouTube, Twitch, ads, etc.), the user can simply search the web for the game, find the website, click "buy" and proceed.

Probably a bit more work than publishing on Steam but not that much. Seems worth it for large budget games or games that are very popular in a specific niche discoverable outside Steam.

>Is there a reason why game developers can't just sell a key on the web using Shopify/WooCommerce/etc. and distribute the game files publicly via a CDN?

It's not that easy; Steam brings down dramatically complexity of self distributing and it offers a plenty of Web 2.0 features e.g. reviews, discussions forum, connecting with friends, playing with friends, chatting with friends etc.

It's like asking why Amazon exists when sellers can have their own store page and distribution channel....

Some games that I know that are popular and not on Steam are for example Starsector and Escape from Tarkov. Game devs choose not to publish their game on Steam either because their game is not finished and they do not want to get negative reviewed to the ground and/or they think their game's brand is so strong that they do not want to pay 30% tax to Steam. For example Minecraft most likely wasn't on Steam because of aforementioned reasons.

Some games do that (Escape From Tarkov), but it's rare because it's just not worth it.

Two words: chargebacks and fraud. Keeping this to a manageable level requires scale.

Some devs do essentially do that in addition to Steam. You're missing a million and one features on top of simply running the game itself if you restrict yourself to that, though.

Stardock's (then Gamestop's) Impulse, Paradox' GamersGate, IGN's Direct2Drive are all competitors that are about as old as Steam.

(Or at least were, these days I don't think they are much more than Steam keys resellers ?)

Epic Games Store is just as atrocious as it was years ago. It's not about time.