For the same reason most large corporations go to the "cheapest bid" to get the most money in their own pockets. Look at Wal-Mart, a privately owned company making it's product purchases mostly if not all out of the country.
Most third world countries live off a fraction of what we here in the USA live off of. Therefore cheap wages.
This has continued to get worse since I started with United Cable in Casper in 1979. I don't see and end to it myself.