My hovercraft is full of eels

I do quite a bit of travel and I've always wished that travel agents would send my itinerary as an electronic invitation but they never do. In fact none of my customers do either. Actually, come to think of it, pretty much no one does. So why don't more people use ICalendar and VCalendar when sending out meeting or appointment invitations?

At one point in my career I was forced - against my will - to use Lotus Notes and yet one thing we all did was send each other meeting invitations. However, rarely if ever have I seen them sent outside of an organisation. This seems rather strange to me as almost every email application I can think of supports one or other of the formats: Mac Mail; Outlook; Thunderbird; Eudora; Evolution; even Lotus Notes!

For example, Mail+iCal on my powerbook allow me to send and recieve invitations. My calendar is automatically updated when an invitation arrives and updates are automatically sent out when I change one that I've initiated.

Have I missed something or is there some reason I have to manually create and update all my appointments?