Question

Why do people think the earth is flat

Answer 1

They are living in the middle ages back in the 17th century. We are in 2017, their spirit is with them in the middle ages and they communicate between centuries to say such thing. 
I hope this wasn't a serious question xD!! 
Have a nice day :) 

But people believed the Earth is flat because everywhere you go you see a flat ground when we are supposed to be seeing a curve in the ground here and there. But is just our eyes, and we cannot see across a horizon for ever. if the Earth was flat we would see China standing from the United States with a good telescope. 

Answer 2

People think the earth is flat because it seems hard to fathom that you can still stay on the earth if you're on the North Pole. This theory, that the earth is flat, was well known for hundreds of years. The vikings were the first to figure out that the earth is not flat, though Christopher Columbus gets the credit.  
Hope this helps! Have a happy holiday!