Yeah the queen can move everywhere, but she's "predictable". The way a knight moves makes it more difficult to defend against IMO.
Another is that a queen, like a rook, can reach any unblocked square in at most two moves. A Knight can require up to six moves!
Add to this the ability to move and attack diagonally like a bishop, but without the restriction of staying on the same colour of squares...
The queen is immensely powerful.
However, once a player becomes more proficient, not only the obvious superiority of the Queen, but even of the Bishop (other things being equal, Bishops are thought to be actually somewhat more valuable, especially as a pair) starts becoming apparent.
Even grandmasters fall for knight forks in blitz games quite often.