If weight loss really is as simple as "burn more calories than you consume", why is exercise necessary instead of just counting calories?

You can just count calories and if you are in a calorie deficit then you will lose weight.

But if you want to lose fat, which I think should be the ideal goal for everyone, then counting calories is not enough because you will not only lose fat but muscle as well.

Not to lose muscle, you need to do two extra things:

1. Weight lifting type of exercise. It could be a body building style, Crossfit, Orange theory, calisthenics and so on.
2. You need to make sure you eat enough protein daily somewhere between 0.8 and 1.2 grams per body weight in pounds.

Following this will make you the fittest self of yourself and will guarantee you will not be skinny fat.

By the way, don’t just focus on the scale. Use also the mirror to see body composition changes weekly. The scale is a good tool but it does not tell the whole story of your diet or weight/fat loss.