Valve's 30% is high, sure. But you're not seeing the total cost of selling a game.

And yes, I've done this before.

Besides the user count, besides all other factors. Digital sales are kinda hard.

You need to offer the actual game. If you're selling an indie game that's a few hundred megs, well you get to go sign up for a service to deliver it. Could be as simple as a google drive link, but because this is business use you get to pay business prices.

Are they charging a flat rate per month, per gig? Per download? Some combinations?

Now there's updates and patches that need to be delivered. Same deal as before, but also now you need to handle the actual patching. Do you ship one big patch that checks for previous patches? Small individual patches that your users have to figure out what one they need?

Does your game have multiplayer? Well damn have fun with that.

What about support and refunds and GDPR stuff? Gotta factor all of that in too.

Now we get to do payment processing. You get to pay a company to accept payments on your behalf because you are NOT doing that yourself you WILL get stuck on inane and silly laws.

That's part of it. Paying steam 3 bucks on my 10 dollar game to handle ALL of that? Yeah that's fair. Could it be cheaper? Sure. a lot of things could. I don't spend months on a game and then cheap out on the most important part: sales.

My time is valuable and worth 30%