Steam gives you so much for the amount they take. imagine if you will, selling fruit. you get to advertise on the worlds biggest platform, and they handle all of the shipping, making sure it's packaged in boxes, providing a way for customers to provide feedback in a single spot, quite a few smaller qol things (trading cards, achievements, inviting friends via steam instead of port forwarding, or "goodies beyond the fruit for customers" in this idea) will handle support for a variety of issues (as in refunding if the fruit went bad, feedback avenues, if they didn't even get their fruit after definitely getting charged), and you just make sure this platform gets the fruit to give to your customers. they want 30 cents on the dollar (or just 30%) of the purchase to handle all of this for you.
Why should they allow you to get around them getting their cut? certain games such as EVE and GW2 were around for however long before they went to steam, and were able to negotiate being off platform (more like partners getting help with larger reach, that... frankly they didn't need, versus platform). BitCraft frankly, is not proven like those titles. I'd personally be upset, because it'd mean BitCraft would be incentivised to up their prices on-platform the 30% to compensate for "lost profits". I use steam a lot for many other games, so my wallet which would be able to purchase those items, now cannot because they up-charged because they want to avoid the "totally evil and dreaded 30% tax".
Also... nearly every platform is like that, they all take a cut of the profits... it's how they operate, because there is no such thing as a free lunch. Steam, for what it's worth, is one of the better places to go, some of the most bang for the buck, so to speak.