Why are the magnetic stripes on the sea floor parallel to and symmetrical across the mid-ocean ridge

New crust forms along the ridge when old crust splits and magma rises to fill the gap.

The mid-ocean ridge is a volcanic feature caused by the spreading apart of oceanic plates as part of plate tectonics. The sea floor at the ridges are uplifted by rising magma. The rising magma is part of a process of heat transfer from the interior of the Earth to the surface known as a convection current.