Your question is a good one. You are right in that the basic premise of weight loss is balancing the intake of calories with the output. Calories ingested that are not used by the body are stored as fat. It seems simple enough. But, as we all know, if it were that simple, nobody would be overweight or underweight.
Many factors enter into weight loss, including the psychological, the biochemical and genetic.
Psychologically, some diet gurus feel that the desire for sweet-tasting food and drink can be overcome by abstinence of such. Using naturally sweet foods such as fruits tends to help overcome the desire for manufactured sweets, after a time. By using artificially sweetened soft drinks, you are perpetuating your desire for manufactured sweets such as processed bakery foods, according to some dietitians. Carbonated drinks can be bloating. Bloat adds to the desire to overeat.