Why isn't Bishop vs Knight considered to be a material imbalance?

they're both worth 3 points so it's balanced.

that's like someone purposely losing the bishop pair to create imbalances in the position.

Assigning a value to the pieces according to the positional considerations would take a lengthy mathematical equation that would change every move, and with every potential move.  This is done quickly with an engine, but to try to do so in a live game would be crazy.

Knight and Bishop have approximately the same MEAN value (3¼ times the mean value of the Pawn), and it's the Bishops pair which adds something to this set of pieces' value relatively to the Knights pair (2 Knights ≡ 6½ Pawns; 2 Bishops ≡ 7 Pawns).

All this is well known, and based on statistical studies of the results of chess games played at the highest level with all possible material unbalances.

Reguarding the characteristics of Bishop and Knight, beside the obvious speed and global mobility differences (the Bishop is a long range piece, hence faster, but the Knight can reach 64 squares vs the 32 available to the Bishop), a crucial thing that is often missed by the casual observer is the versatility of the Knight, which can move in 8 different directions vs the Bishop's 4.

Actual results show that Bishop and Knight are overall equal in value (if not in pairs), and it's the features of the position which define which is stronger in any moment of each game (as it happens for each and every piece on the board, really).

Furthermore, in my humble opinion, these two pieces being at one time so different but so equal is the single feature in Chess having the highest responsability for its strategical variety and richness.

Another thing about knights ,  no other piece benefits as much from " centralization " .  

One thing I can say for sure about bishops is that when facing a lone king, I'd rather have the bishop pair than any other combination of minor pieces.  Give me a knight and bishop against the lone king and I'll take the draw.  Actually, since I've improved so much since the last time I looked at this ending, I should take another stab at it.  Come to think of it, I should probably review the 2 bishops ending since it's been about a year.

Thanks, Rando!  Now I got something to occupy my time.

Well, maybe the queen too...but queen centralization doesn't always have to be in the center always really, if you catch my drift, in many openings as white, a queen on g4 is very pesky and is extrememly well placed.

But yes, I agree.

What's nice about the queen is that it's always good -- open positions, closed positions, even in the corner it's not bad at all

Also note that the bishop is on the average taller than the knight on most chessboards. Definitely a compelling argument.

Some players evaluate the bishop with a slight edge as you can force checkmate with 2 (opposite coloured) bishops, or bishop and knight, but not 2 knights (barring extenuating circumstances... if the defender has a pawn on the board its doable). Bishops rule open positions and knights are better in closed. In endgames, the bishop has the reach but can only ever cover half the board - the knight takes longer to get there but can pick off pawns no matter what the square. If material's equal then those games really come down to king activity.

Take an empty chess board and place a knight on a1 ( or any corner square ) and it has only 2 possible moves and then place it on any central square ( the extended center also works ) and it has 8 possible moves so its power is quadrupled by centralization . The queen's power isnt even doubled and no other piece comes close to having its range/power quadrupled by centralization . The rooks arent helped at all by centralization , on an empty board . The bishop and knight both benefit more than the heavy pieces do from centralization . 

Knight vs Bishop is considered an imbalance, although a positional one rather than a material imbalance.

The problem in that kind of reasoning, as always, is "on an empty board"...

That only makes the point stronger of course, but knights always control 8 squares, when other pieces can be and are blocked when the board is not empty.

I have wondered why people always think by putting a numerical value to a piece.

The aim of the game is to mate your opponent, or save the game by a draw. And the strength of each piece is dependent on the position and what you are trying to achive with that piece.

For example I usually like to exchange the bishop for an opponent's knight when I am planning to put a knight on d4/d5 (just so it cant be traded off). I also make that exchange when my opponent has to take with the pawn that is infront of the castled king, hence creating a weaker king and a hanging pawn.

In other positions where there is a diagonal pawn wall which blocks off one of your bishops, I will exchange that pieces first chance I get.

I will also give up my knight for the bishop, if my opponents bishop is really active.

SO yeah....

Quoting richard Reti, Chess advanced course or something like that (from memory) :

There are positions where a queen is worth more than a knight. But if we have to think in every position whether the queen is worth more than a knight, it is impossible to play well. That is why there are general principles, even if they are broken sometimes.

I assume it's not referred to as a material imbalance is because they are both "worth" (assuming the pieces' scopes are similar) 3 points, so there is no "material" imbalance.

There's definitely a positional and strategical imbalance however when it's N verse B. 

because they are worth the same.

They are both equally useful. But I always value Knights. If you put your opponent in check with a Knight, it forces them to move out of the way, because they can't block it. Also Knights are really good for, I believe "forks" is the term for it. Where you check the King and as a result are able to take another piece as a result of him having to move out of the way.
And aside from anything else, they are the coolest looking pieces on a chess board.

Why are there so many Bishop vs Knight forums. The simple answer to all the B vs N battles is:

It depends on the possition!!!!!!!!!

I don't think that answers the question of why there are so many B v N forum threads.