30% is only the standard because Apple set it so high. All of the others just copied them - "Apple charges 30% so we will look reasonable doing the same". It's basically the figure you can get away with if developers have to publish in your store.

When developers don't have to publish in your store, you have to lower your cut:

https://blogs.windows.com/windowsdeveloper/2018/05/07/a-new-...

That is why Microsoft and Apple are so keen on pushing the Mac/Windows stores, and on making running unsigned code a huge pain in the arse. They want that sweet sweet 30% cut on desktop too.