You're losing close to 4lbs a week, that isn't sustainable in the long run.
Usually when people talk about losing weight, they have a lot to lose, it can take a long time. Restrictive diets that last months/years aren't realistic, the average joe doesn't have the willpower.
When people talk about dieting, they talk about living an unrealistic lifestyle. Most people on diets will cut out everything that is considered bad, this usually doesn't last long, hence they crash and burn after a short while and get no where.
The reason people talk about it as a lifestyle change as opposed to a diet is losing the weight and also learning how to eat properly. A diet is great to lose the weight but what happens when you've reached your goal? Do you resort to how you used to eat? do you keep on the diet permanently and adding slightly more calories to maintain? You could do that, but again it's doubtful people will be that strict and frankly it's a little obsessive.
Yes you need to be in a calorie deficit to lose weight but it's lot easier to achieve that by making small changes in your life that will impact it in the long run, not the space of 4 weeks.