@tongdtbds3 In this case at least, people say it because its true.
Unless you do extended bouts of strenuous, cardio-based exercise pretty much every single day, it will not result in a meaningful calorie deficit. And even if you DO do that your body will downregulate other calorie burning processes, further blunting the calorie burning effect of exercise (which is already low). Pilates and resistance training and yoga are even less effective than cardio. And adding up the numbers from fitness trackers proves nothing because (a) they're notoriously inaccurate and (b) as noted above, exercise isn't purely "additive" in the way people think as I noted above re: compensation.
Exercise is amazing and great for health. Everyone should exercise regularly in a way that they enjoy. And yes, it "helps," but exercise is a relatively minor factor in weight loss. Its regularly discussed on this sub, however, as if its a viable weight loss modality on its own, which it is absolutely not.
So that is why people say this.