Epics is much lower because theyre trying to entice devs, but they are the anomally in the sea of pricing.

Epics trying to win market by enticing devs instead of working on features for the consumer, thats their market plan. Epic wasnt the only platform to have lower than 30% cut. Discord sold games at 10% cut, itchio is similar. Devs essentially debate of the baked in features of the platform and its audience is worth the 30% cut(the existing community, game review system, steams controller api, steam workshop, steamvr). Even just the client. ESPECIALLY to Linux users, on a consumer POV, ask yourself about ease getting to use the native client. Valve offers steam natively, and does a lot of work making the consumer end (and developer end too) easier on linux. EGS for example doesnt even run natively on linux, and requires a 3rd party launcher to run. People tend to take for granted all the things Valve has done for both the consumer and Developer.

Discord massively failed to get users, and devs saw little market in it. Epic takes advatage of their position using unreal engine, and offers some devs money upfront for exclusivity, something certain audiences on PC absolutely hate.

Users use steam because it simply offers them the best user experience. There are a ton of people who just buys their games directly from valve and not a 3rd party site. To a consumer, money's not necessarily the problem on their end, and they dont see the 30% hit that developers take. Something good for the developer is not necessarily good for the consumer and vice versa, and many people make that mistake and conflate that to be the same thing when it isnt.