Of course personal travel is different than personal health. Missing a flight/cruise/hotel is annoying, but not dangerous to your health. I don't think it is the same to say that it is just as equally irresponsible to not take care of your health, as it is to not buy travel insurance.
And again, not everyone even knows such a thing exists. I know that before my first cruise, I didn't know all the rules. And I didn't read all of the fine print either. It was my honeymoon and I was more concerned about lots of other things. I made it there and everything was fine, but I don't think it would have been irresponsible of me to not buy travel insurance because I didn't know such a thing existed. And not everyone researches every single aspect of ever vacation they go on, so you can't just say that you SHOULD know, because not everyone does.