It's not the physical scars that are the problem, it's the mental and emotional scars. Encounters with deep horror and evil change you in ways that set you apart from other people. Frodo could have stayed, but if he had wanted to be a part of the community, he would have had to act as if he'd never had those experiences, and to do so eventually kills you.

My therapist speaks of "shiny world people" and "dark world people". The "shiny world people" are people who can't really grasp the depth of horrors that are in the world -- they say things like, "you can't live without hope" or "I know that good will triumph." When told of horrible stuff, like life in a concentration camp, at some level, they don't believe it. It's not real to them.

We "dark world people" are people who have been through enough Hell that we know that there is no horror that the human mind can imagine that is not happening somewhere. When we hear of some terrible thing happening, we know in our gut that, yes, it is real and it really is that bad, and in fact, the truth is probably worse.

In my case, it wasn't physical (well not much), it was emotional. I grew up in the power of people who hated what I was and wanted to erase me and replace me with someone more to their liking. Everything I did (or failed to do) was not just wrong, but WRONG, and people were constantly impressing that upon me. They seemed to think that if they made me miserable enough, I would stop being who I was and start being what they wanted. I spent years thinking every day about how I could kill myself, and some 60 years later, I still have frequent bouts of suicidal ideation. I self-diagnose myself as having complex PTSD, and this part of the description definitely applies to me:

[They] ... may feel defined by stigma, believe they are nothing more than their trauma, worry they're always in the way or a burden, or they may sense they're just completely and utterly different from anyone or anything around them - they are alien.

I've sometimes described is as feeling like a space alien -- like ET, but not as cute. Or that there is an unbreakable invisible wall between me and the rest of humanity, and I'm on the side that's like a dungeon. (Dank stone walls, dirt floor, no light except what comes through the invisible wall, etc.)

It's also what happens to people who have been through a war; you see it in the war veterans who can't seem to adjust to civilian life. The things they have experienced are seen as impossible, unreal by people in civilian life, so they can't talk about what they've experienced with anyone.