How come none of the immediate royal family ever die of cancer or any other horrible disease?

i know they used to but they all live to a good old age now, im only on about the main ones, it makes me wonder if there is a secret cure for cancer but its kept for them, maybe to keep the population down.
Update: like i said IMMEDIATE royals, also margret was a heavy drinker and smoker, im not anti royal it just seems like they dont suffer like joe public.
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