Because people like to discover another food, the history and culture of another country and people want to forget their life for 1 or 2 weeks.
Here the top 30 of the most visited countries.
https://www.youtube.com/watch?v=WJlJ_vUL2q8

I made 2 of these 30 countries      (and you ?)
but not just any !   France the first       and USA

But did you consider me like a tourist if I live in France    

My next travel/goal : Japan