[user5994461]

It is a few percents. All online marketplaces, steam, apple store, airbnb, booking are 20 to 30% commission.

It takes at least 10% commission to be able to run a company of this type. It's possible to run with as low as 5% in some niches, given a very lean operation and a billion dollar scale to amortize fixed costs. Eventually most of the company will be devoted to customer support and billing related tasks.

There is really not that much that can be shaved off. Small studios could never make a platform any efficient. A small group could spend all their time working on distribution and billing, never shipping an actual game. It's a really good deal to just use steam instead.

For AAA games it's annoying to pay 30% and they're rather make their own platform, that's why steam is dropping commission to 20% for top selling hits.

[MattRix]

You say this, but there are already services like itch.io and humble that charge ~10%.