I live in Pakistan and I just bought Lumia 1520 but there is not Pakistan maps in it, Why?

Nokia Here (and their Navteq partner) don't appear to have detailed Pakistan Maps... There is also a large hole the size of Japan in their map making world... The issue gets brought up from time to time, yet there appears to have been no fix yet...

Try a web search on the subject and you'll see it's been ongoing for some time.

Indications are you may get some offline nav functionality by downloading adjacent areas in Asia.
 
i am unable to find Pakistan maps to download them at settings->applications->maps

You should contact MS for unavailability of maps of your region. How they forgotten? Despite Pakistan's maps are listed as available on windows phone site.
 
I guess you should mail Microsoft and ask them this question. They will be in a better position to answer your question.