Because 30 minutes is 2.08333% of your day.
The idea of the book essentially would be to that people should exercise for 30 minutes per day and burn as many calories as they can during that time period.
And since 30 minutes is only a little over 2% of your day there really isn't any excuses for why people cannot use 2% of their day to get fit.
"Are you willing to use 2% of your day to get fit
and get the body you've always wanted?"
And if you answer yes, then it is time to start using that 2% to achieve your goals.
Once you say yes the next thing you need to is schedule the time.
That means you need to find 30 minutes in your day to exercise and do nothing but exercise (and maybe listen to music or watch TV while you exercise).
It also doesn't matter so much what exercise you pick - as long as it is burning lots of calories it is going to be beneficial. The list below is some popular / fun exercises that you can choose from or come up with your own.
Notes - I listed the moderate versions of various exercises because some people may not be able to do the vigorous exercises for 30 minutes. All calorie calculations below are for an 191 pound individual, which is the average weight of an American Male - since 67% of American men are overweight or obese, the average number should be closer to what many people are in weight. If you want more accurate calculations there are many calorie calculators available online and also apps for your SmartPhone.
Boxing Punching Bag - 258 calories in 30 minutes.
Calisthenics, Moderate - 195 calories in 30 minutes.
Cycling, 12-14 mph - 378 calories in 30 minutes.
Cycling, Leisurely - 172 calories in 30 minutes.
Fencing - 258 calories in 30 minutes.
Hatha Yoga - 120 calories in 30 minutes.
Hocky or Road Hockey - 350 calories in 30 minutes.
Jogging - 304 calories in 30 minutes.
Pushups/Situps/Etc - 206.5 calories in 30 minutes.
Swimming, Moderate - 264 calories in 30 minutes.
Weight Lifting, General - 149 calories in 30 minutes.
What this tells you is that if you want to do something more aggressive you might to choose activities like boxing and road hockey, because they burn a lot of calories because they are heavy cardio activities. Cycling, swimming and other cardio exercises also burn lots of calories and this will make a fundamental difference in burning fat.
THE AFTERBURN EFFECT
The Afterburn Effect is a metabolic condition whereby your metabolism is raised far above its normal rate and your body burns roughly 500 calories more over the next 24 - 48 hours and you end up feeling really energetic and positive about yourself.
It is caused by raising your blood pressure up to roughly 85% of its maximum potential - which is more easily done through cardio exercises that get your blood pumping fast. Thus the really high cardio exercises like running, vigorous swimming, boxing a punching bag, sports like road hockey, soccer, etc... they can vastly increase your calorie burn because they are more likely to trigger the Afterburn Effect.
Thus lets pretend you spend 30 minutes every day boxing a punching bag. Each time you do this you put some serious effort into it and get your heart rate up to 85% roughly.
Math wise for someone who weighs 191 lbs, this will mean they burned 258 calories and will burn an extra 500 calories over the next 24-48 hours.
In a week they will burn 5,306 calories - or roughly 1.52 pounds.
In 30 days 22,740 calories - or roughly 6.5 pounds.
In a year (364.25 days), 276,101.5 calories or 78.9 pounds. Possibly more if you are eating healthier as you progress. (Less if you aren't eating healthy, or if you are doing exercises that build muscle through strength training or endurance-strength training because you will gain muscle weight during the same time period.)
Don't worry about your weight so much - go based on how you are feeling physically. Your weight can deceive you if you are gaining muscle weight while losing fat at the same time. If you want to measure progress use a tape measure around your middle.
Find ways to have fun while exercising. Sports like tennis, cycling, boxing, archery, fencing, martial arts - these are all great ways to exercise and have fun at the same time. You will not lack for motivation if you are doing an activity that you love.