They distribute games, which is something in addition to creating and publishing.

Whatever percentage they use is based on an average across wildly different games. A large game with frequent updates doesn't need to pay steam for the work on steam's end each update. They don't need to pay for each tine someone downloads their game, or for the ongoing costs to upgrade steam over time to continue supporting their game. They have a set percentage per sale so they can easily calculate how many units they need to sell to break even.

If the game's sales die off they don't need to pay for steam to continue support. At any time they can use the popularity of a new release to renew interest in past releases without any extra requires work. When game sequels blow up, the publisher doesn't need to do anything to get sales money from new sales of the prior versions. The prior games are just there, waiting to make the publisher money.

How much value do you think any distribution platform provides?