Question

why do archaeologists think that many people who lived in Mehargarh were hunters to start with, and became herders later

Answer 1

because it can be simply understood as at first people would learn hunting as they saw other animals hunting and slowly slowly they observed plants and than become herders

Answer 2

archaeologists have found different types of remains from deeper levels of excavation at mehagarh. Remains from  deeper levels show bones of wild animals.This shows that people were still hunters.Remains from upper levels show bones of domesticated animals.This shows later on people became herders.