While I agree in principle (to share with those invested in your wellbeing), might do the same, and am otherwise unqualified as a fairly young person without living parents to refer to, I think I can understand why someone would prefer to postpone the revelation of that kind of news to someone they loved, especially their kid, until they knew for sure.

Edit: specifically, it would either be to avoid causing others undue stress on account of a potential false alarm OR to postpone the decision of revealing a terminal diagnosis until I was ready for all the emotional difficulties that tend to follow.