I've written before about escapism on more than one occasion. Suffice to say (for those who can't be bothered clicking links or opening new tabs) I'm one of those people who thinks that escapism is one of the most noble of human pursuits, perhaps the noblest of all, and it should be trumpeted to the heavens as one of the finest features of the hobby which you and I enjoy. The fact that 'escapism' is treated as a dirty word by the intellectual elites in our societies is rather a sad indictment of prevailing sentiments of the age.
This is why, generally speaking, I think that fantasy and SF are the genres which gamers tend to love. At the moment I'm running a quasi-historical gonzo-horror affair and it's nice - nothing to complain about - but I've found a certain sense of malaise setting in when I sit down to plan the next session. The real world is a wonderful place, but there is something lacking when using it as the basis for an RPG campaign; the creativity that goes into that process feels impure somehow, and pales in comparison to the flights of fancy the imagination can go on when unrestrained by "reality".
I believe this is the real reason for the prevailing popularity of familiar brands such as D&D, Traveller, and the like. Escaping to Timbuktu is all well and good, but escaping to Waterdeep or the Planet Zong is better; why have cotton when you can have silk?