Because the human race has a stupid conception of its own indomitability, which shows up in its popular fiction, its political propaganda, and its sociopolitical and socioeconomic policies. Even in its religion, as most tend to have as part of their mythology that the entire universe was created especially for us, and in some cases that we on earth were literally the center of the universe.
It also shows up in our role playing games. Even in games where humans aren't the top dogs (like Midnight, for ex) the essential premise is often that humans will pesevere through all these problems and there is a sense that inevitably there could be the happy ending as someone (the hero PCs, usually) will restore the balance and bring humanity back to its rightful position of dominance.
Its because humans are stupid and arrogant to the point of hubris, and its something that leads us in our arrogance to follow the wrong leaders (the ones who tell us "our country is number one!! its just common sense!" and drown out more intelligent men with more intelligent solutions to our problems.
In game terms I would find it most interesting to see a campaign of, say, the Forgotten realms, where all that talk of "Waterdeep" and other great cities and human realms are so much








, the only surviving humans are the dales and a handful of other places most races don't want, and humanity is rightfully near extinction in a world where almost every race is vastly more powerful or vastly more prolific than they.
But humans being humans, the humans of the dales live with the myth that everything is fine and dandy, that there are humans in various lands and great human empires.
Start your player characters as young adventurers in the dales (preferrably pick people who are already familiar with FR and will assume you are playing a standard game of it) and let them start thinking that the FR is as presented in the FR campaign book.
Only after a few adventures and some horrific example of the rapid extinguishability of human settlements should the PCs realize that the Atlas of the realms part of the FR campaign book wasn't truth, it was propaganda.
Nisarg