That's becasue the copy effect makes an exception to power/toughness and replaces them with specific values. In such a case, any characteristic defining ability setting the exempt or overwritten characteristic is not copied.

707.9d When applying a copy effect that doesn’t copy a certain characteristic, retains one or more
original values for a certain characteristic, or provides a specific set of values for a certain
characteristic, any characteristic-defining ability (see rule 604.3) of the object being copied that
defines that characteristic is not copied. If that characteristic is color, any color indicator (see
rule 204) of that object is also not copied. This rule does not apply to copy effects with
exceptions that state the object is a certain card type, supertype, and/or subtype “in addition to
its other types.” In those cases, any characteristic-defining ability that defines card type,
supertype, and/or subtype is copied.