It doesn't work like that.
Game publishers provide a range of services. One of those services is getting your game out to customers. Sometimes one of those services is helping with advertising. Sometimes they help with testing and QA, help passing console certification, help providing customer support. Sometimes one of those services is fronting money.
All of those services have a cost.
If they are providing you money up front to develop your game, you will make nowhere near a 50/50 split at first. They are taking a tremendous risk in investing in your project with no guarantee of reward, and that is expensive money. Exactly what you negotiate in that situation depends on how much money you need and what you are bringing with you to the negotiating table. A game that is basically complete and needs funding for the final push is radically different from a game that is a proof of concept and seeking funding for the entire project.
Steam has been a source of frustration with the 30% take since they launched, but originally Steam was doing far more than providing a download link and payment processing to justify the cost. Steam curated games, provided distribution, customer support, anti-piracy and anti-cheat systems, assisted with marketing, assisted with launch services, and helped with so much more. Others followed suit because they could. These days Steam and other distributors are having a very difficult time justifying their 30% rate as the number of services they provide has dropped over time.
For standard publisher services like distributing through Steam, distributing through Live, distributing through Epic or Origin or the Apple Store, you'll pay whatever rate is standard for the service. If you're talking about it here you aren't a large enough business for them to bother changing their standard agreements.
If you're looking for services like fronting money, advertising and promoting the game, featuring the games on their stores, those services will cost based on the details you negotiate with them.
