Why do some parents refuse to take responsibility for their health, even when they have every opportunity to?

I genuinely want to understand if others have faced something similar.
I’ve noticed a pattern,not just in my own family, but also among relatives and friends. Some parents repeatedly complain about their health, go to doctors, get diagnosed, and are prescribed treatment, but they don’t really make any effort to understand their condition or change their lifestyle.
They won’t exercise, won’t improve their diet, won’t follow through consistently, and often ignore advice until the condition becomes severe. Then, when things worsen, the responsibility naturally falls on their children.
In my own case, after my father passed away unexpectedly, my younger brother and I took on a lot of responsibility. We cleared a significant amount of family debt, got our family settled, and have always tried to support our mother in every way possible.
Now she has health issues that could potentially be managed with regular exercise and physiotherapy, but despite repeated conversations, she isn’t taking the initiative. My brother also ignores his own health despite already showing signs of high cholesterol at a young age.
What makes this emotionally exhausting is that I know if these issues become serious later, both of us will have to put our lives on hold to manage the consequences,something that might have been avoidable.
I’m not expecting perfection. I know aging brings health problems, and not everything is preventable. But what I struggle with is when someone has the ability to help themselves and simply chooses not to.
Has anyone else dealt with parents (or family members) who seem to have a “whatever happens, happens” attitude toward their health? How did you handle it without constantly feeling guilty, frustrated, or responsible for decisions they themselves are making?
At what point do you accept that you can’t force adults,even your own parents,to take care of themselves?