I guess because in general it's not a good use of a piece if it's the only thing a bishop or knight is doing.
If it's already there, or if it's an active square anyway, then sure, but I can't really imagine taking the time to maneuver a knight or bishop just to block a file unless it's to save my king from mate or something. The usual antidote to an opponent seeking play is:
1) generate counter play in a different area (usually the best option whenever possible). If they're gearing up for the queenside, then your moves are ideally setting up ideas for you in the center or kingside.
2) Contest the file with a rook or queen yourself (before the opponent gains full control of it). At the very least you'll be able to trade a pair of rooks (maybe two!) which will lessen their attacking potential.
Blocking is direct defense / passive defense, and is usually the worst choice (in dealing with any kind of play from your opponent). When active pieces tie down defenders, it's usually easy to shift the attack to new squares (or even the other side of the board) and win something that way.
A favorite tactic of the opposing queen and rooks is to slip down an open file and wreak havoc behind the pawn structure, unravelling it from behind, or marauding about looking for mating opportunities. However, a useful way to prevent that is to block an open file with a knight or a bishop, supported by one or two pawns propping it up. This then usually forces the opposing player to pay too high a price for breaking through this barrier to the area behind, and even if the plug is removed by a piece of equal value, often when the pawn moves in to take it, it closes up a more vulnerable open file than the one it opens by capturing.
But I wonder why this tactic is so little discussed? Shouldn't it be as common a topic as 'forks,' 'x-ray attacks,' 'removing the defender,' or 'pins'?