Why does a field test involving a magnet often indicate a genuine meteorite when testing for internal clues?

Because most meteorites contain iron or nickel metal content which causes attraction to a magnet

One of the simplest preliminary identification tests for a potential space rock relies on magnetism. This characteristic is directly tied to the chemical composition of the debris. Meteorites, especially the Iron and Stony-Iron categories, incorporate significant amounts of iron and nickel metal as remnants from their parent bodies. These metallic components are ferromagnetic, meaning they are readily attracted to external magnets, such as a common refrigerator magnet. Most ordinary terrestrial rocks, composed primarily of silicates, lack sufficient concentrations of these magnetic metals and will typically fail this simple magnetic screening test.