Since when? Since at least CalDAV since Apple proposed it. Actually, probably back further than that as they support compatibility with the global email system, too.
“iCloud Mail: iCloud Mail does not use end-to-end encryption because of the need to interoperate with the global email system. All native Apple email clients support optional S/MIME for message encryption.”
I personally have no need for CalDAV and CardDAV being interoperable with the world. But, my assumption is that for those in mixed system environments, the ease of calendaring from one device to any other device is worth it, so Apple supports it. For most things, when folks indicate “There is no reason…” there usually IS a reason, it’s just that the person that mentions it is opposed to recognizing the reason.