Why do parents do this?

So ive across this vídeo, showing on how a "family" (if you can call those monsters family) celebrating their child 17th birthday, saying on how he still fits in his 12 year old shirt, hes almost an adult and he has to wear his 12 yr old shirt because of his physical disability, like genuinely, how can you keep someone imprisoned in a body like that?? Id bet if the guy could make a coherent sentence, and someone asked him if he wanted to be alive, he'd say no, and that he wants his suffering to end, sorry if i sound apathetic, but its just the truth, no one wants to live like that for their whole life. I seriously wants to know what goes in someone's head to let someone with a serious disability, live like that, and no im not eugenic or anything is just that letting someone suffer their whole life because youre trying to be "empathetic" or "compassionate" is extremely selfish