I honestly think the reason people do that is to convince themselves that they aren't going to be the next to die. It's a sort of built-in defense mechanism - "Well, the deceased must've done X, which I don't do, so I'm not going to die like they did!" Like dying of cervical cancer ("She must've slept around, which I don't do!"), AIDS ("a filthy druggie or a *exceedingly impolite term*), or even a car accident ("they could only have been speeding or texting"). Pushing the responsibility onto the deceased instead of crazy, random happenstance makes people feel more in control of their own lives and deaths.
A high school friend's little brother died right after reaching 21 of complications from MS. If anyone had suggested it was anything less than savory, I'm sure anyone who had ever met him would have rained vengeance down upon that idiot's head - he was one of the sweetest, kindest, most loving and patient people I had ever met.