Because Amazon chooses that to make a point.

Notice how it's totally fine to buy goods in the Amazon app? You think that Amazon couldn't have used their leverage to make the same case for Kindle if they thought it was worth it?

I'm not saying there is no value to owning a platform, it's just not as extreme as people make it out to be.

My understanding is that they don’t want to pay a 30% cut to apple on every subscription or booked purchased