The location section of the iCal event window is designed for use with server based calendars which have a listings of locations. For example in a company the location field would be a meeting room and when the person making the event types in the name of the meeting room it shows as "in use" for anyone else who might try to book that same meeting room.
While you can type in standard mailing addresses, these are not the type of "location" that iCal (or the iPhone's Calendar app) was designed to interpret.
If you would be interested in seeing a feature like the iPhone Contact app map ability, let Apple know:
The iPhone is a little bit of both. The location field of the iOS Calendar app can be used just like in iCal if the calendar you are using is part of a calendar server which has locations loaded. Otherwise it is simply a field to type in locations "Grandma's house, Mike's deli, etc".
If I had to guess as to "why" the location field on the iPhone does not function as a "tap to map" field like in the Contacts app it is because the Calendar app has no way to know if what you are typing in is a "real" address (5959 No Street Your City, Your State ZIP) versus a generic address reference like "mom's house".
In the Contacts app there are pre-defined fields for street, city, state, zip etc. so when you tap on these locations, the app can easily figure out what it is searching GoogleMaps for. In the case of the location field in the Calendar app it is just one big field with no defined areas to type in the various components of an address.
All that said, your suggestion has merit. Let Apple know you'd like to see this feature added:
"It is inconvenient, but until they get it fixed, here's a simple alternative. Instead of typing the address into the location field, type it into the notes. It is then a clickable link to Maps. Maybe they'll get things more integrated soon."
I have tried that but only the street number and name are linked, the suburb remains simply as text?