Why does ATT charge $30 to set up a new phone on an existing account. and is there a way to avoid it?

First off all, here is not a att support and every cellphone carrier charge some money when activating a new phone with a new line within our account there is no way to avoid that because your are taking a new service even if you are using the same SIM card from your current phone you have to pay those charges

Because they can, because people will pay it, and because it’s part of the requirements for connecting to the AT&T cellular network. And of course, because it’s profitable.