yeah, but you have to remember, "Adventurers" (and that's what every PC is) are a step above the rest, so while it may be common among adventurering warriors to have the 18 str, it is not common amongst the general populace, or even average soldier. and adventurers, as has been said many times, make up a very, very small portion of the world's population.
Also, as Dorg suggested, by DnD standards, 10 is average. Beyond 20 in any attribute is considered superhuman.