The biggest reason they’ve been tightly coupled historically has been event notifications and invitations. It’s a lot easier for one email client to both create the event in the calendar, and send the event metadata (.ics file) to the invitees.
Nowadays, it’s honestly much simpler to have them entirely separate, at least for personal use. My partner and I use a shared NextCloud calendar which works well on both iOS and Android using CalDAV. Much simpler than Google/microsoft/icloud’s sharing options.
Americans will do anything but build townhouses