Is it because we have no written/archaeological evidences of a civilization in pre-literate Southern India? The classical "sangam" literature of the Tamils purportedly traces its origin all the way back to 9000 BC. Are their claims purely mythical and exaggerated or is there substantial evidence pointing towards societies/empires back then that simply get neglected in popular discourse?
We have few primary sources because the main medium of writing was on dried palm leaves which don't survive well in South Indian humidity. For primary sources we have to rely on relatively rare documents engraved on metal