maybe to avoid getting their legitimate email servers banned by other servers since they host (i.e. being exploited) a growing number of spam accounts.
https://support.apple.com/guide/icloud/use-hide-my-email-in-...
I think I've also seen this in Mail.app but that's not shown on this page.
UPD: apperently this supposedly only work if someone message you first. So you still cant spam from aliases.