For myself, if I focus on diet and not worry about exercise I find that I have to eat very little if I want to lose more than a few pounds. So little that I'm miserable all the time. Forever. On the flip side, if I exercise[1] I not only burn calories but increase muscle mass which burns more calories. Other people seem to do well with a focus on diet and not so much exercise. Weight loss should be about losing weight in a healthy, sustainable way, however that works out for the individual. To continue doing something that's not working because it's supposed to be The Right Way is counterproductive and demotivating. I think most people can find some balance between eating healthier/less and exercising more, in a way they can keep up.
1. I've only had two effective exercise regimens: resistance training (weight lifting) and martial arts. If I try to jog, walk, or whatever I will become more and more irregular and eventually quit. Other people love to run. Whatever works.