What you did there is subtle, but you're comparing apples to oranges. "payment card companies" charge 2-3%. It's platforms like Steam, Play store or App store that charge 30%. Those are not comparable.

I'm not arguing that 30% is the right number, but the analogy is very flawed. Ask anyone what % of their game sales comes from Steam vs their own website/itch/humble/epic. Let alone the dozens of other things that you get with a few lines of code (cloud saves, free in-game text/voice chat, workshop, marketplace, free download and easy patching, achievements, etc) and without having to host a single server or maintain anything. How much would it cost both in development but also hosting cost to code all of that yourself?