Because it's self-contained, you can easily stuff it into lots of different kinds of envelopes without disturbing it. You can throw a MIME wrapper around it and email it over the Internet. You can also wrap it in little infrared photons and send it over an IrDA link. If you require MIME to separate the various parts of a vCalendar object it becomes much more difficult to deal with in toto. MIME becomes sort of a cancerous envelope and it requires significant surgery extract the letter intact.
Lets consider another example.
Suppose we formed an IETF working group to work on the difficult problem of exchanging CAD information. We have this super object format called DXF that will do everything you need already. You could I suppose, but it would probably be a very poor move. Suppose I want to mail you a GIF image of my dog. The different elements of a multi-part MIME document are typically processed by separate applications.