Not like any other app store does take 30% except for some high volume games/publishers.
Apple does the same. Hell they seem to have custom rules for each of the app devs (according to Linus and Luke from LTT: I believe this clip contains most of it. They recently talked about it again. Essentially they developed the app payment like Netflix. Apple said "No, that's against our rules" and refused the submission of the update. Meanwhile Netflix supposedly still had the same communication for a long time.)

Same goes with Google and probably a number of other external stores.
Amazon seems to take up to 20% depending on the item (Source: sell.amazon.com/pricing.

At least Steam does provide a forum, community features and the update framework and infrastructure.
Personally I would be happy to take the offering over maybe needing to host and maintain the tech stack myself. Now mind you, maybe some other dev would rather do it themself and maybe wish to opt-out of the ecosystem. That is totally valid.

(Warning/Disclaimer: I only heard about that. I do not have first hand experience!) Apple for example takes a percentage for processing a payment and offers an invoicing system. Some may like that. Others could maybe negotiate a better deal with another provider and maybe even offer tools that integrate better with their existing accounting and ERP software.