A bunch of reasons
- The Jeep has a significantly larger engine, 4.0 is a lot more than 3.4. The straight six in the Jeep (it's not a V6) is also an older design than the V6 in the Pontiac, which makes it somewhat less efficient. The Cherokee is also heavier - the base 4 cylinder 2WD Cherokee was probably only about 3300 lbs, but a six with 4WD will weigh a lot more. Aerodynamics also play in, the Jeep is shaped like a brick compared to the Grand Am, it makes a big difference on the highway. Then you have the 4WD system itself - even if it's switchable between two and 4WD, the extra weight and mechanical complexity play in here too. Gearing may also be a factor, I'm guessing the Jeep might have somewhat shorter gearing than the car.
Put them all together, and you can see why there's a big difference.