More commonly, the devices are moved internationally, frequently to Mexico or China, sold as parts, or wiped and used off the network. So that new $1,000 iPhone gets sold for ~$30… It doesn’t seem like buying a $400 ticket to a music festival to steal phones would be worth it, except when you can grab 100 phones (back of the napkin math = $2,600 profit), it looks pretty good.
Multiple teams of pickpockets flock to festivals to steal your phone and whatever else they can get their hands on. It has unfortunately become big business and is only getting bigger.