Question

Once graphene is formed why don't it convert back to being graphite

Answer 1

Diamond is metastable at standard temperature and pressure. While graphite is more stable, there is a rather large energy barrier (breaking many covalent bonds) that has to be crossed before carbon atoms can be rearranged from the diamond lattice to the graphite lattice.

At room temperature the atoms don't have enough energy to cross the barrier, but if the temperature is raised high enough then the material will spontaneously transform from the less stable allotrope to the more stable form.