A bit more on the "why"...
Yes it's 20.
Because level 20 is also the highest you can ever get in a "real" paper&pen-campaign based on the standard AD&D or D&D rules.
Theoretically, that is. Since in fact you rarely ever get beyond level 14 in paper&pen games. And even that requires you to play for a looong time -and- you need to have a DM who also works towards high levels.
Of course you also have to remember that there is a huge power difference between a
PC game and a "
real" paper&pen roleplaying campaign. In any "normal" (i.e. non-digital) roleplaying adventure, a level 15+ chara is already close to godlike. He will have a sh*tload of spells/day, a breathtaking armor class and plenty of other stats to boot.
Handling such a superhunk is a really tough job for a DM, as the player party is so powerful that they are almost unstoppable without resorting to... let's call it "certain DM privileges".
So... that's the
original reason for the cap.
The "
Epic" levels (ranging from 20-40) were introduced -much- later ... and only as a bit of play-candy for highly seasoned and trained roleplayers/DMs. Since if your players aren't really experienced and cooperative, controlling an "Epic" campaign is like riding a rabid dragon...
Of course a game like NWN offers far less problems when it comes to high chara levels. :P In a PC game, no player will b*tch if, for some reason, he can NOT break through the door of the farm house across the street. There, the only problem is basically to scale up the enemies stats so that they dont crumble on the spot when being attacked by the players private little level40 demi-god.
Anyway... if you ever play a level40-chara in NWN, you will very quickly notice that 40 is DANG well enough in terms of power.
-Zel