Why is fuel usage discussed per time, not per distance?
Unlike with driving, in aviation the major variable when planning fuel consumption isn't how far you're traveling but how long you're traveling: Distance traveled in aircraft is a derived value (calculated from groundspeed and time - groundspeed also being a derived value affected by many factors including wind).

A pilot may not readily know their groundspeed (it can be quite different from airspeed), but at a given power setting they know an engine will burn a specific quantity of fuel per unit time, therefore fuel consumption is generally expressed that way (in gallons per hour or pounds per hour), and if they know how long they've been flying they can reliably estimate how much fuel they've burned, how much is left in the tanks, and how much longer they can keep flying before running out of fuel.



To illustrate why this is superior for flight planning consider the following (extremely simplified) flights fitting the following profile:




A 200 nautical mile trip
An airspeed of 100 knots
A fuel burn of 10 gallons per hour
A 25-gallon fuel tank.


Scenario 1: No Wind
With no wind the flight will take two hours to complete.
The groundspeed is 100 knots.
The aircraft will burn 20 gallons of fuel.
The fuel efficiency is 10 miles per gallon.


  In this scenario we land with 5 gallons of fuel left in the tank - that corresponds to 30 minutes, which is the minimum legal reserve for day VFR flight per FAA regulations.


Scenario 2: 25 Knot Headwind
With a 25 knot headwind the flight will take about 2:40 minutes to complete.
The groundspeed is 75 knots.
The aircraft will burn 27 gallons of fuel.
The fuel efficiency is 7.5 miles per gallon.


  In this scenario we run out of fuel 2:30 into the flight. Running out of fuel is generally considered to be a Bad Thing: The passengers complain, and the FAA asks a lot of uncomfortable questions.


Scenario 3: 25 Knot Tailwind
With a 25 knot tailwind the flight will take about 1:40 minutes to complete.
The groundspeed is 125 knots.
The aircraft will burn 16 gallons of fuel.
The fuel efficiency is 12.5 miles per gallon.


  In this scenario we land with almost an hour of fuel still in the tank - a very comfortable reserve. 


On longer flights, and in slower aircraft, the effects of wind can be extremely pronounced, and a 5 or 10 knot difference from the forecast winds can mean the difference between arriving at your destination or having to add a fuel stop.