Why don't we consider fat people attractive?

[Grey Acumen]

Obesity is generally unhealthy, and health is the predominant standard upon which any physical attraction is going to occur.

Another factor is also money. Way back in the dark ages and such, it was considered beautiful to be fat. Why? Because being fat meant you had the money to afford to eat, whereas just about everyone else survived from meal to meal, no real chance to get fat at all.
In modern times, the reverse is actually true. Fast food is the cheapest thing around, it's health food that costs you an arm and a leg. So people who are fat now are subconsciously equated to being poor.

Another factor, which is my personal preference is that it help hint at how well you take care of yourself. I can't really bring myself to invest a personal connection to someone who doesn't take care of themselves properly, thus not just fat, but anorexia, drug use, heavy drinking, and to a certain extent, promiscuity, also fall under things are not attractive to me.