Like I said, that is an oversimplification. There are many other factors that play a role, like the body working against the weight loss and lowering metabolism and increasing appetite, as I already discussed. There are also psychological factors and environmental factors that can have a big impact on weight loss.

Just ignoring those makes losing weight more difficult and means you have to do it entirely based on willpower. If you are one of those people for which these other factors play a big role it becomes very difficult to keep the weight off in the long term just based on willpower. This can be the case, for example, if you have an eating disorder, very high stress, untreated medical conditions (e.g. hypothyroidism), not enough money to buy healthy food, problems with the body signaling hunger or fullness, unsolved emotional issues, and so on.

Of course there are some people who just eat too much and who can just lose the weight easily by eating less. However, especially for people who are very overweight, these factors will play a role. They hear they should just eat less all the time and if they fail they are made to feel like a failure themselves and as if they have a lack of willpower. This is not the case. For them losing the weight is more difficult and the underlying issues should be addressed. You cannot just generalise like that and apply what works for you to everyone else.

Edit: please find sources for my claims in my comment below.