There are many reasons why people live unhealthy lifestyles, most of which are out of one's control. General unhappyness (for whatever reason) can lead to over eating, over eating can lead to more unhappyness, which leads to continuously eating unhealthy etc etc.
Most people don't want to be fat and unhealthy but ...[text shortened]... Are the ways our brains are wired not also determined by genetics, upbringing and surroundings?
That's an abdication of responsibility.
Perhaps there are individuals who are condemned to be fat because of genetics, but most people are fat because they don't make the choices and enforce on themselves the discipline necessary to stay healthy.
Obviously, factors such as poverty correlate with obesity, but just as we don't excuse the criminal because poverty caused him to become a criminal, you can't just excuse all fat people because of their circumstances. most people get fat because of poor choices. We can alibi for them all we like, but that's not going to help them and there's no way we can legislate those poor choices out of existence.