Why was agriculture and not industry the leading economic activity in the United States in the early 1800s?

The industrial revolution hadn't happened yet. It took place in the late 1800's and early 1900's. The Industrial revolution created opportunities for people to find more jobs that required less difficult physical labor for better pay. That is also when large companies got their power. they could afford to hire many workers to do thigns like work at an assembly line.

Before this, the vast majority of people were subsistance farmers who made little or no profit on their crops, and ate most of what they grew/ raised, themselves.