The magnetic field between two magnets decreases as they move apart because magnetic fields follow an inverse square law with distance. This means that the strength of a magnetic field is inversely proportional to the square of the distance from its source.
When two magnets are brought close together, their magnetic fields interact with each other. The magnetic field lines of one magnet link with the magnetic field lines of the other magnet, creating a combined magnetic field in the space between them.
As the magnets are moved apart, the distance between them increases. According to the inverse square law, doubling the distance between two magnets will result in the magnetic field strength being reduced to one-fourth of its original value (1/2)^2 = 1/4. This decrease in field strength is due to the spreading out of the magnetic field lines over a larger area as the distance increases.
In simple terms, the magnetic field strength decreases as the magnets move apart because the magnetic field lines become more spread out and less concentrated.