A piece's value is not merely a function of its mobility. A board can be loaded with pawns that impede a Bishop of the same color, making it a "very bad" Bishop and a piece you would trade for a Knight immediately. One long series of pawn swaps, and now the Bishop might be poised to dominate the Knight in the ending.

As I discussed in my paper here there are values other than the "mobility/safe check" that also factor into the equation. On page 90 of this paper:



There are a few things worthy to note at this point. These are not exact ratios universally accepted by the chess-playing public. The work of Taylor in 1876 provided a foundation upon which players could experiment and adjust the relative merits of the pieces over the years. In this respect, Taylor’s equations will compute semiconditional values for the pieces (Katsenelinboigen, 1997, p. 53). In this fashion, we differentiate these data from conditional, unconditional, partially conditional, and positional values that are computed in a variety
of ways by the contemporary chess master at various stages of the game.

This is explained in more detail in the paper, and in my reference to Dr. Katsenelinboigen's book.