Well first of all, many of the people that adventure all the time....are dead. Adventuring is dangerous dangerous work. PCs often make it because they have DM fiat, the convenant that says most DMs will not give you encounters so crazy your going to get vaporized without a chance. Most NPCs don't have that, like in real life stuff happens and they get killed off.
Second, leveling after 13 1/3 combats isn't that unrealistic to me. Your telling me if I take a guy straight out of boot camp and have him fight in 13 1/3 different engagements he's not going to be better at shotting his gun, better at controlling his fear, and better at gaining cover and staying out of the way of bullets? Heck, if a normal guy gets into 13 fights in his life, he's certainly going to be better at fighting than a guy who never has.
Third, keep in mind how powerful a 1st level character is compared to the benefit you gain for the next few levels. You start out with max hp, if you assume average rolls that's as much as you gain for the next 2 levels. You get +2 to your best save, you get max 4 ranks in a skill. It will take you 4 more levels in order to "double" the skill you've acquired in that "background training." Wizards get 1st level spells, they won't get a higher grade of spell for 2 more levels, barbs gets rage, rogues SA, etc. So I think 1st level does well to show the background training a character has gone through.
So I don't think the leveling speed is so unrealistic, no more unrealistic than what high levels characters can do in the first place.