Because the fact that they are fat shows they have major mental flaws and issues already, so it isn't a "shame" that you don't get to fuck these nut jobs.  Even if they were thin, they have proven that they are mentally ill dumpster fires of human beings.  I guess some men think it's cute to try to fuck anything that walks, but men with standards don't want to stick their dick in crazy.  If you would be willing to fuck one of these women just because they lost some weight, we hate you too for letting these insane people think they are fuckable and functional humans.  If no one would fuck them and flatter them, they would have no choice but to face reality.  Basically the people who go on about how pretty a fatty's face is and that it is a shame, those people are enabling it to continue.