Multiple reasons. Most common and overlooked is that you have brakes that are dragging. Make sure your calipers are free and the slides are not frozen. Check the rear brakes and install a new hardware kit. Do a proper brake bleeding, make sure your hoses are in good condition and that all wheels move freely when the car is jacked up.
If you have a check engine light, you need to pull codes and find out what is going on. Make sure you check engine light is functional and has not been disabled or burned out by a previous owner. Sometimes there are codes without a check engine light, so double check that as well.
Clogged catalytic converters can also cause poor fuel mileage, as well as a poor state of tune. As stated above, check the plugs, wires, and distributor cap and rotor for excessive wear. Change dirty air and fuel filters, and make sure the tires are properly inflated.
Lastly, double check that your odometer is accurate against mile markers.