Network effects & economies of scale - lot of people living together in close proximity in Tier 1 cities, creates a bigger local economy, where everyone is involved in working to fulfill one another's needs.  
  
 Hence more jobs and bigger business market for everyone, and so higher earnings for everyone wthin the area.

Staff Engineer

But Why every day people are moving to mega cities? Why not people move from mega cities to tier3 or tier 2 cities?

Project manager

Design Engineer

Founder

Per capita income is more in mega cities why only those cities?

Why are tier 1 City people richer then tier2 and tier3 cities?