GamblerGORD53, there is more to the world then your limited life experiences. You sound the the owl in this video;
The mechanical efficiency of a drive system is defined as the ratio of the power output to the power input; often measured in percentages. In a vacuum on a flat plane, all things being equal, Dr and IGH are close to being equal, but overall the IGH requires 8% more inertia in some gears to get going and has little to no input momentum (when you are coasting with a freehub/wheel there is little resistance bringing the drivetrain up to speed; with a IGH you feel more resistance as you bring the internal workings back up to speed. Another issue is not only are IGH do not lend themselves to quick releases, add to that the rear wheel weight and linkage to the hub and rear wheel removal can be problematic. To answer all three of your question, the answer is weight. When all things are not equal and you start climbing, weight becomes a major factor. I don't race anymore, but most of my bikes are 20-22 lbs and riding a 30lbs+ road bike feels like I'm riding in mud. Depending on the number of gears this will add 1 to 3 lbs to the rear end. The weight distribution of a bicycle is usually around 60% rear. With a 22 lbs bike, a rear 3 lbs difference (7-8 gears) would increase 7% weight to the rear, which would likely effect medium to high speed fork stability.You also have the fact the gearing of an IGH is fixed, while with a derailleur it can be changed to fit the ride (the widest gear spread is not always the best option).