I think she's also heard it all a million times before, so it doesn't hit home. She's probably been told she'll get diabetes and have a heart attack since she was 16. And, in her mind, because it hasn't happened (yet), it means she can ignore it the next time.
You see this when something new pops up, and she does seem scared by it. The first time she finds out she could have a blood clot, she got mad because people were scaring her by saying it. Then she goes to the doctor, and because it wasn't a clot, that fear is immediately cast aside forever. Or sleep apnea. That was scary, but her incredible power of denial managed to suppress the fear within a few days, and now sleep apnea/sitting up when she sleeps isn't an issue.
It's probably a lot more complicated than this, and I definitely agree she lacks the cognition to realise what a big fucking deal eg. diabetes can be. The denial is fascinating to see in action.