Why it's not unfair?

As we know when a pawn reaches 8th rank or 8th row it generally promotes into a queen. Why there are no restricrion against promotion? say why it's not restricted to Knight or Bishop, Here I mean why we get the powerful queen simply by promoting a pawn as it makes the opponent worse in the game because we are simply a piece up in the game that too the powerful piece. Even chess laws allows us to have two queens (say one original and the other promoted). Isn't it unfair? Sometines opponent need to sacrifice his bishop or knight or even rook in the fear that pawn will get promoted to queen. Why it is so? Please give me a proper clarification.

No rule in any game is unfair if it applies to everyone equally. Unfair by definition means that one side is given an unearned advantage over the other. Everyone can promote to a queen so by definition it’s fair.

If you mean "over-powered" rather than "unfair"... yes, it's SUPPOSED to be over-powered.

Chess began as a war-game, a simulation of battle. In battle... especially in ancient warfare... allowing the opponent to drive right through one of your flanks and get behind you is almost always catastrophic.

The promotion rules mimic this: it is important to maintain your own formation while you break through the opposing line.