Maybe because in our society being fat is being not good-looking, and nobody is gonna convince the people of the contrary despites the ' good looking big woman ' community or such things.
We live by the society, as long as the society will work that way, it's up to us to fit into it.
The only thing that I'm not alright with is people thinking that being fat is bad because it makes you ugly, being fat is bad because it's unhealthy and shall it be considered.