I have never seen this on Android, and it definitely seems like developers don’t care about the 30% cut on Steam, even though sideloading is fairly easy on Windows. How many Steam games can you buy direct, let alone get a discount for?
The majority of PC games aren't sold through Steam even though it has a large installed base and provides some level of promotion. Meanwhile many of the popular Steam games are made by Valve itself or are free games paying Valve 30% of nothing, implying that the large majority of third party paid game developers didn't think the 30% was worth it when given a choice.
That is literally already the case for many apps. Many offer a better price outside of the iOS ecosystem.
And why are you drawing the goalposts at EVERY developer?