This argument is so odd and it depends on what you take away from it.

Is the message "some people still get sick, so what's the point".....that's very misguided.

The point is that all the lifestyle actions you take, give you the best chances, but there are no guarantees with anything in life. You're still playing the odds, but trying to stack them in your favour as much as possible. If you prepare for an interview, you're not guaranteed to get the job; but you prepare to give yourself the best chance you can. You can't decide to do nothing because the outcome is not guaranteed. Concluding "why even bother" is such odd thinking.

Sorry about your dad though, RIP.