Because apple isn’t in control of apple for data at rest, and that’s the specific risk.
You have to trust control of the device sure, but you cannot trust cloud data - almost at all - between subpoenas from over eager LEOs and break ins from criminal and state hackers
That's not really true if Apple also holds copies of your iCloud decryption keys. If they want to access your data, they already have all the necessary components.
That is literally the thing that this announcement changes.
I see that Hacker News has plummeted below Reddit in the "bothering to check the link" stakes.
> One must understand that E2EE is used when you don't trust your service provider to handle your data. In other words, the adversary in your threat model is the service provider - and in this case, Apple. And what good is that encryption, if Apple obviously can do almost anything with your device?
Let me re-phrase, by giving Apple control over the keys, you give control over the data to whoever controls apple - which is non-zero (Eg. LEO), and whoever may gain control (security vuln).