It’s simple math.  Only 1 to 2% of customers change carriers.  There is no way any carrier can financially offer deals to the other 98%.  

Carriers can also write off a deal to a new customer as an expense to add new business, they can’t with existing customers. 

If you want a deal, keep your eyes open and shop outside the carrier.  Best Buy and Samsung have preorder deals all the time.  They may also be trade in deals.