People who are obese or grossly obese often aren't just eating a "little extra" here and there. Imagine more like "extra pizza a day" or "eat a cake once a week because that seems like a good idea".
That is why weight loss is hard to achieve. If you are hungry, it's hard to--over an entire day--avoid eating 300 extra calories.
1Kg of human fat is around 8000kcals give or take. 50lbs is 22Kg. 176000kcal. It's 586x "extra slices of pizza", almost two years of overeating every day.
It's easy to say "put down the pizza," but pizza is just an example.
300 calories is also 1/4 a cup of rice (50 cals), 1/3 service of chicken (100 cals) an egg (78 cals), a teaspoon of olive oil (40 cals), and a toast (75 cals).
There are lot of reasons its hard to cut weight. Measuring calorie intake to 300 calories is hard enough. But also, its easy to eat a bit more at each meal if you are hungry (and it adds up).