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.
This is the same mentality as the fashies wanting to remove all Gay content from the Internet because some kid might turn gay. Even though absolutely no one would choose to be gay let's be fucking honest.
I don't think anyone is looking at fat cats and going gee I need to overfeed mine for clout!. And if they were they're already crazy and that cat had no chance in hell.
Otherwise let's remove all fat humans off the internet too! Might encourage over eating tsk tsk.
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.