Final answer:
People in Southeast Asia predominantly live in coastal regions due to the economic benefits from transportation and trade, the agricultural productivity of fertile deltas, and the rich cultural exchanges facilitated by historical maritime activities.
Step-by-step explanation:
The reason why many people in Southeast Asia live in coastal regions is primarily Access to seas and other waterways which enables transportation and trade. Coastal regions offer an abundance of resources and are crucial for economic activities such as fishing and transportation. The fertile deltas created by rivers such as the Ganges and Brahmaputra and the agricultural opportunities they provide are essential for supporting large populations. Furthermore, the history of Southeast Asia has been shaped by its geography, where the monsoon winds facilitated trade and cultural exchange that have led to the region's rich diversity. Coastal living in Southeast Asia is deeply interwoven with the area's economic, cultural, and geographic realities.