Is there a specific reason why Apple releases software updates for BOTH iOS and macOS at the exact same day?

There are other reasons for simultaneous releases (like features that require updating all devices on an iCloud account), but this is almost certainly the most important one. I don’t know how Apple handles putting security patches in beta updates, but they don’t announce the fixed security issues until the final release.

Now of course most users don’t apply updates immediately, but this at least gives more security conscious users the option to do so.