Magnets can follow inverse cube law but gravity follows inverse square law, why?

It has to do with the distribution of field strengths. If you suppose a sphere encloses a permanent magnet, and you measure the magnetic field over the surface, you will notice that its strength depends on longitude and latitude, because of the poles. If you do the same with the electric field of a static point charge or gravitational field of a mass, you notice that the magnitude of the field strength is uniform. Otherwise, you'd weigh differently on different parts of the Earth.

A concept called flux is created for understanding this. This flux is the opposite concept as heat flux. What this kind of flux is, is a surface area integral of the field. For a uniform field, it turns into a simple multiplication by the surface area of the mental sphere. By physical laws, the total flux of the closed sphere doesn't depend on what size of sphere, as long as both spheres enclose the same mass. For this reason, we can explain that gravity "spreads out" to become the field on a larger sphere.

Since surface area increases as the square of radius, gravitational field strength decreases. The same is true with an electric field due to a point charge. For a magnetic field of a permanent magnet, the math is much more complicated because of field concentrations near the poles.