Because they don't control those. Apple could choose to only allow users to access websites that pay them a bit 30% fee, but users would notice the web was turned off on their device. They don't notice when the app store does it.

I don't think people would notice if Apple just made the website behind a paywall. Most people are not going to be aware that they can access the same content without paying a fee to Apple. They may only even have an Apple device to access the internet, so they'd just see it as normal