The term SouthEast Asia is a modern term developed only during World War II. The area was known in Europe as the Spice Islands while Arab traders called it the "land below the winds"
The area consists of two parts , the land mass below China and East of India to the tip of the Malay peninsula, and the archipelago that includes Indonesia and the Philipinnes today.
During the Ice age the islands to the South were connected by a land bridge. Even with the end of Ice age it was reasonably easy to cross this area of water.
The whole area is traversed by a series of forbidding north south mountian chains that made travel very difficult. That resulted in the isolating of civilization that developed in the river valleys from one another. The result the region never developed the cultural or political cohesion that we see elsewhere in the world.