Partly because airline sales take place significantly before the product is delivered, so unless the airline is significantly hedged (which not many are nowadays) the money paid for the fuel being burned next week was paid by the customer some months ago and has been sitting on the books as unearned revenue liability since then.
If airlines priced themselves fuel neutral and charged passengers surcharges at departure time when fuel prices rose, the passengers would scream bloody murder. The fuel surcharge developed as a means of recovering the revenue shortfall on today's tickets by taxing future tickets. This principle holds true if fuel prices either stabilise or go down, but so far the trend has been steadily up, up and away.
Most airlines on 15-30 day pricing cycles are expecting to be hit with a 15% rise in Jet A-1 costs on December 1. Expect to see a rash of fuel surcharge hikes right around that time.