Although 30% is a lot, it is not just a cut 'to list a game'. If you were to not go through Steam and instead do it yourself, you will need to plan for things such as the data transfer for everyone trying to download the game. Those costs add up. Large companies like Bethesda will be fine, but I'm just explaining that there is more to it than a fee for being listed.
Yeah sorry I was being a little blunt to make the point, there is of course overheads with hosting the downloads and the transfer of that data and the setup to allow ease of revisions, the forums and workshop use and the launcher integration. Still 30% would be a lot for companies such as Bethesda but makes much more sense with smaller companies, start ups and similar where that overhead to revenue is not an issue but the support Steam offer is valuable then.
There is little incentive for larger companies to utilise Steam in today's age.

