TLOU -- so where are all the infected monster things? Where is the military? Why are the unaffected living in those quarantine militarized prison areas .. apparently you can just go live in the forests.
You've seen where the infected are, in population centers where people were concentrated (thus giving them new people to infect and continue the infection).
You've also seen what's left of the military. FEDRA, the organization that was running the Boston QZ is one such example.
You've also seen why people are living in the QZs, from both perspectives. People flee to the QZs for safety (from the infected and from the outside raiders like those depicted attacking Frank's compound). There's also resources and some of the closest things to civilization inside the QZs. Additionally, once you're inside the QZ, FEDRA doesn't let you leave. That's the whole bit about Joel, Tess, and Ellie having to sneak out of the QZ (and why Joel and Tess' skills as smugglers were valuable). This is also why they've shown groups like the Fireflies trying to overthrow FEDRA in Boston, and this other group who successfully did so in Kansas City (Pittsburgh in the game). Are there some people living outside of the QZs in the forests or their own compouds? Yeah, but the vast majority of people aren't survivalists. Most people don't last in the wilderness, especially not for 20 years. That's why you saw Frank begging for help (just 3 years into the infection) at Bill's gate, and why the rest of Frank's group had died during their trek from Baltimore to Boston (and as Frank explained, the Baltimore QZ had collapsed).
