The price of oil is dropping. So why worry about 'peak oil'?

The price of oil is volatile.  It's affected by economic conditions across the globe, by political upheavals, by the weather, by bottlenecks in the supply chain, by people's spending priorities, and many other things.  So the price goes up an down dramatically.  What we need to keep an eye on is the price over time.  And over time the price has risen remorselessly.  Since 2001 the annual rise has been between 18% and 25%.  This indicates that supply is not keeping up with demand.  The big question is: why?