When Bob is the lead protagonist of a work, his presence is essential to the plot. Accordingly, the rules of the world seem to bend around him. The very fact that he's the main character protects him from death, serious wounds, and generally all lasting harm (until the plot calls for it). Even psychological damage can be held at bay by Bob's suit of Plot Armor. -tvtropesIt's natural to think of Plot Armor as a bad thing, a kind of invulnerability with no in-world explanation.
But I think it's not as bad as it seems at first sight. Suppose the possible world where Bob's story happens were actual. There is a selection effect as to which people we want to hear a long real-life story about. First, their life has to be interesting. One way for a life to be interesting is for the person to face a lot of danger. Second, their life needs to be sufficiently long to tell a long story about. Third, we don't want to hear too many depressing stories, so we don't want a story about someone whose life completely falls apart. All of this makes it likely that even in the real world, stories like Bob's are going to be likely to be told.
In a world with billions of people, we expect some to have multiple unlikely hair's-breadth escapes. And we'd like to hear stories about them. It's unlikely that escapes this narrow happen to Bob, but not so unlikely that they happen to someone.
So it's false to say that Plot Armor has no in-world explanation. If we imagine the story as being told by an in-world narrator (perhaps an implied one), we can give an in-world explanation in terms of selection by the narrator.
Of course, when the unlikeness of the escapes reaches the point that we wouldn't expect anyone to have them even with the population being as large as the story portrays it to be (Science Fiction about a whole populated galaxy will have more latitude here due to a much larger population to work with), this is problematic.