Because he's a good fighter with some brilliant attributes. He has blinding handspeed, an excellent jab and goes to the body well. He has ALOT of heart also and is very good offensively, but he absolutely 100% does have a glass jaw. This is irrefutable.
He has no defence and low or no ring IQ and gets caught flush, thats his problem. Anyone hits the dirt when caught absolutely flush with a power shot, there is no such thing as an unbreakable chin in boxing. Thats irrefutable. If Khan was going down and staying down from nothing shots then Id agree with you, but he doesn't.
He has no defence or ring IQ and does get hit flush, agreed. He's been knocked down and seriously hurt by guys like Michael Gomez and Willie Limond at domestic level aswell though. His chin problems date back to his amateur days aswell, he definitely has poor punch resistance. He can hold a shot when he's properly conditioned (working with Alex Ariza) but he will always been one punch away from disaster.
He wasn't hurt against Gomez, don't exagerate to enhance your point. There is a world of difference between being put down, getting up and then stopping the other man to being knocked down and kept down by a feather fist. The latter is the definition of a glass jaw and they are rare in boxing for obvious reasons....Eric Crumble pretty much has a glass jaw. Every fighter is just one punch away from disaster, thats what makes the sport intriguing