As consumer I very much agree with you, but as game developer 30% is abysmal amount of money. Imagine you're indie developer or owner of a small 3-10 people studio that finally released reasonably successful game:
1 - Let's say you invested $100,000 of your own money for vertical slice and managed to find a publisher to give you $200,000 to complete the game.
2 - Ignore that you had some failed games before, but this time you let's say sold 100,000 copies for $10 each average. 100k sold is a big success really.
But here is the math:
1 - Valve got $1,000,000 as gross revenue for 100,000 sales.
2 - Usually 16% is VAT and immenient refunds. So now $840,000 left.
3 - Now Valve took their 30% cut. $588,000 left.
4 - Now your publisher took $200,000 to recoup invested money. $388,000 left.
5 - Now publisher split remaining $388,000 by honest 50/50.
Now your company sold 100,000 copies of a game, but only get $194,000 gross income as royalties. And if you will make any profit you'll likely pay at least 20% corporate or divident taxes so yeah at best your profit gonna be $155,000.
So you did all the work, somehow managed to fund it, worked on game for a year and got $155,000 while Valve made $252,000 for payment processing and CDN. Steam do not provide marketing - it only boost already successful products.
PS: This is best case scenario. Usually your publisher will also recoup whatever expenses they had on their end for marketing and whatever.