why do nginx process run with user nobody

Is there a reason why they are not running as www-data ?

Yes. You most likely haven't specified the user in your nginx config.

User Directive: http://nginx.org/en/docs/ngx_core_module.html#user

syntax: user user [group];
default:    
user nobody nobody;
context:    main

How to run nginx as a particular user?

You can specify the user/group that nginx runs as, in the nginx config.

This is an example of what an nginx config might look like (notice the user directive):

pid                 /path/to/nginx.pid;
user                www-data www-data;
worker_processes    1;

events {
   worker_connections  1024; # usually 1024 is a good default
}

http {
   # more code goes here
}

Simply update your config and then reload or restart nginx and you should be good to go.

Of course you should choose the user that works best for your system, in Debian/Ubuntu there's a www-data by default, so that's a sensible choice.

The master process is run as root, then nginx will setuid()/setgid() to USER/GROUP. If GROUP is not specified, then nginx uses the same name as USER.

By default it's nobody user and nobody or nogroup group or the --user=USER and --group=GROUP from the ./configure script.

You can edit nginx.conf and set user to www www;