One of the biggest reasons might be, because "chonkers" can be considered an animal abuse, as overfeeding your pet is not healthy for said pet, and causes them to suffer from inability to live their life to the fullest, and most likely they will strain their internal organs to the point of failure. The knowledge of this has reduced the popularity of real life "chonkers", as people actually feel empathy towards animals, instead of considering them as cheap "entertainment".
Still, the premise of such a community is the display of unhealthy cats, and it inarguably will encourage some people to encourage and continue unhealthy treatments of their cats.
I remember the first time I stumbled over that sub on r/all and posted a negative comment not unlike yours I got downvoted into oblivion because apparently every chonker poster was just about to save that cat's health.
People can choose how much they eat. Pets can't decide for themselves and will always eat if there is food in their bowl. For people, a lot of the times it's an addiction and will definitely lead to health issues, so even in that case it's something to worry about. Food brands occasionally need to present goals in order to reduce sugar, salt and fat in their products. Don't get fooled by appearance, pets need to be taken care of and the same goes for people.