Every teacher MUST simplify things first.
The student will get bored otherwise, if he is forced to swallow all the eventualities immediately.
We start chess by studying open openings (Scottish, Italian, etc.), because this is the fast way to exemplify the pecularities of every piece: bishops need long diagonals, rooks need long files etc.). Only later we learn that there are different ways to develop pieces: the opening complexes 1.d4, 1.c4, 1.f4 ...).
Castling early is more often true in a position with one open file in the center, because then the rook (Rf1 in case of 0-0, or Rd1 in case of 0-0-0) is ready to attack over the open central file.
With closed files (or sometimes even with half-open files) castling may not give your attack an immediate benefit.
Moreover, if Queens are exchanged early in the opening, then the best place of the king might be in the center (and not in the corner).
Here is an example of an opening variant played by Kasparov in 1981 against Romanishin, and later often employed by Kramnik:
1.d4 Nf6, 2.c4 g6, 3.Nc3 d5, 4.cxd5 Nxd5, 5.e4 Nxc3, 6.bxc3 Bg7, 7.Nf3 c5, 8.Be3 Qa5, 9.Qd2 0-0, 10.Rc1 (Apparently, White is not keen to hurry with short castling ?) cxd4, 11.cxd4 Qxd2, 12.Kxd2 (Another possibility is Nxd2, opting for Nb3 and short castling later but here White has other intentions...) Nc6, 13.d5 Rd8, 14.Ke1! (One of the most amazing moves in openings I have ever seen. The idea of the King maneuvre is to force the Black pieces to less active positions, and to take out the sting of a central break e7-e6. The white spatial advantage is a good compensation for the temporary disruption of the white rooks. Note, recently Kramnik has also tried 14.Kc2 here, in a game in London) Na5, 15.Bg5 Bf6, 16.Bd2 b6, 17.Rc7! Bg4, 18.Ba6! e6, 19.Ng5!. White won later in great style. See:
http://www.chessgames.com/perl/chessgame?gid=1069937
I hope I made myself clear better this time.
Of course, one shall see more games, to convince yourself that games with late castling (or not castling at all) are not merely exceptions.
Botvinnik has played many games where he castled late and instead first restricted activity of the opponents pieces.
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