Because a lot of people don't have the willpower to say NO to somebody else.
E.g., my ex-fiancee's son is quite fat.
And it's quite needless. While we were together periodically he'd get motivated to trim up some, and never complained, had a good attitude, and dropped substantial fat pretty easily and quickly.
After we broke up, and therefore my influence became zero instead of simply secondary, he was I guess 10 and had dropped to about 92 lb and was in fairly reasonable shape (he's tall for his age.) Within a couple of months he blew up to 126.
I have repeatedly advised her on the principle of simply drawing the line at some given pants size: this guarantees not getting fatter than that. At this point he was wearing a 14 Husky (codeword for "fat.") Obviously it's ridiculous for a 10 year old to start wearing a 16 Husky, so, draw the line: do not buy pants bigger than this.
But despite apparently agreeing, pretty sson that's what she's buying.
Then men's size 32 for an 11 year old kid.
I forget at what point size 34. Certainly by age 12. Supposedly she was going to draw the line there.
Now he's 13 and must be on his way to 300. I don't know what he weighs, but I think it has to be over 250. No idea on the pants size but it must be pretty big as he's now a real fatass in the literal sense.
And it is NOT that he demands tons of food or sneaks food or things like that. She CHOOSES to keep buying him bigger clothes, thus enabling him, and keep providing the food.
A lot of people cannot say no to someone they love when it comes to food, or buying them tents for clothing for that matter.
Must be the same story for whoever fed this fellow up to 1000 lb.