First of all, quality matters. You could satisfy your entire calorie requirement with lard, but that would be extremely detrimental to your health.

Also, how our bodies process food is still not well-understood in detail (although there are tons of self-titled experts claiming otherwise). In other words, excess calories == weight gain and insufficient calories == weight loss don't work out just as nicely as you'd think (I think there was an article on HN recently that even pointed out different people have different "default weights" they would find very difficult to stray from in either direction).

Additionally simply not eating that much can be very difficult. Not everyone has the time, energy and money to ensure they eat healthy (and the right things eat the right times of the day) -- although of course for energy this can be somewhat of a catch-22 (bad eating habits lead to lack of energy leading to bad eating habits). And there's also the problem of satiation and rabbit starvation: it's hard to stick with a diet if your body signals you that you're starving and must eat.

As a rule of thumbs, sure, just eat less calories if you're overweight. But good luck sticking with that if you don't have the time, energy and money to experiment with how you can make that sustainable. Otherwise the "yo-yo effect" wouldn't be such a widespread and widely-known problem.

I also like me some moderate energy reserves; it improves almost keeling over from low blood sugar when forgetting to eat or no food is available for a few hours tremendously.