why do people from different parts of the world have different accents?

To them they don't have an accent, you do. They speak their own languages so when they speak another language it is going to be with their own inflections. Just like if you spoke their language you would have an accent to them.

Then you have regional accents like in America. People from the south speak differently than people from the northeast. New Yorkers sound different than people from Boston. I imagine they have regional accents in other countries too.

because their respective languages put different sounds on the letters and some sounds have no use in some languages at all