Well training for that is way more time and facility intensive than most folks can manage.

So the takeaway becomes "oh, if I move around in a pool for 20 minutes every couple of weeks that should be really good at weight management".

Yes swimming is nicely low impact so you can more credibly stand hours a day of that much intense activity and between the activity itself and the energy hungry musculature you get it can make a difference, but that is way way beyond realistic for all but a tiny bit of the population.

Realistically, realistic exercise is good for various things, like joint health (when done correctly), cardiovascular health, insulin response, and tons of other stuff. However it will not manage weight that well nor stave off all the associated health risks.

While i question your kcal count, i agree that swimming is one of the few great exercises for weight loss.

Also what most people consider very long distance running (like at least 20 miles per week, realistically even more) is great. IIRC your body doesn't even start to primarily burn fat until like 30 or 40 minutes into an intense workout and it's slow to burn any.

Pay attention you are at the gym. 90% of the people on treadmills are walking with the vast majority getting off after 20 to 30 minutes. Dont get me wrong. Its better than doing nothing, but its like bare minimum maintenence and definitely not even close to fat loss.

People burn through a small amount of their glycogen stores at the gym. Maybe 100 to 200 calories. Go home and drink a 500 calorie smoothie as a reward and wonder why they aren't losing weight.