Naturally, if you consume 25% fewer calories than you burn, you will lose weight. However, you will be less healthy as a result. There's a very narrow band where you can actually lose weight in a healthy manner. It varies from person to person, and furthermore, you are making yourself weaker and more vulnerable to disease if you are eating less than your caloric requirements.

So you're saying "you're fat, that's your problem, you should just eat less." The issue is that eating less causes its own problems, and for some people those can seriously outweigh the benefits of being thinner. For some people, their ability to metabolize fat is so retarded that what you suggest really isn't feasible. They will likely starve to death before they get rid of their fat.

These people are, of course, rare, but the same issue exists in lesser values for other segments of the population.