I just want opinions on this bc I don't understand. Parents only plz

I'm doing this anonymous bc of the nature of it. Okay so I babysit my 2 year old niece and she hasn't started potty training yet. She's talking but not in full sentences yet. Anyway today I see she has a bad rash on her little lady parts and it's worse than I've seen before. I put her diaper rash cream on it of course but my question is why do some parents not pay attention to their child and seems like they let them get a rash? The parents of my niece I've seen and other members of my family has seen them not change her when she needs it then bitch about her getting a rash or try to accuss someone of not changing her often when I've seen them let her go 3 to 4 hrs without changing her dirty diaper. Do they think it'll make her want to start potty training if she stays in her dirty diaper or what is the reason if there is one? They're good parents but I just don't understand the reason why they let her stay in a dirty diaper for so long and she ends up with a rash. Is rashes bad for her little lady health? I'm just curious about this and it's my niece so I change her often when she needs it to avoid her getting a rash. I don't have any kids myself so I feel I can't talk to them about it bc they might just get mad about my concern for my niece or can I say something even though I'm seen as not knowing anything about kids bc I have none?

Edit: by dirty I mean pee or poop or both and not just wet. they use Johnson and Johnson baby wash the yellow bottle and the bedtime one that's purple. They use parents choice diapers idk if the brand really matters or not. She has been using the same brand for like 2 years so I doubt it's the diaper or her bath wash since they haven't changed. Literally nothing has been changed with her cleaning health wise. They bath her maybe twice a week