Travel time calendar

Why doesn’t travel time for an event in calendar update automatically based on current location?

Because travel time is calculated from the field in the calendar entry called Add Location or Video Call. This does not change with movement of your iPhone.

If an event in the calendar is in close proximity to another event in the calendar the travel time in the later event will be calculated from the location of the first event - provided that the Add Location field is populated.

If you want an estimate on the travel time from your current location open the Maps app and search for your destination. The app will then show you a journey time. From this you can tap on the blue indicator to get Mr. Sulu inside the Maps app to plot a course.

Or you could ask Siri to "Get Directions to [address}" and it will be done for you.