Why gravity pull objects down toward earth?

If there is "gravity" then it is an attractive force . that's the way it works. Einstein hypothesized that instead of "gravity", spacetime itself was bent by mass.

Objects continued on straight paths (when not interfered with by other forces) but the space that they moved through was bent by the presence of objects with mass. The Sun has a very large mass and so bends space a lot more than the Earth or a butterfly but all objects obey the same rules.