Why do calendar invites that I send come from noreply@email.apple

The invitation comes from Apple's iCloud servers, not from your device and therefore does not require a response by email. It is an automated system which interacts with the calendar architecture and not email clients.

When you click Accept, Decline, or Maybe that action similarly contacts the servers to update that status.

Then why does it send an ics file by email

For the same reason that Google Calendars and other calendars do - to have data to put into the recipient's calendar in a standard format. It's not unique to Apple.

The invitation system does include non-iCloud/Mac/iOS users. The ics file is a universal calendar format used on many platforms. I've sent invitations from Calendar to Windows users for example.