Not sure if you've tried lately, but when you pay cash for a burner phone, most retail outlets require a driver license to scan your information into their system. When you activate it, you need quite a bit of personal information.

It used to be really easy, but law enforcement is cracking down on the ability to buy and activate these anonymously for obvious reasons.

Are there ways around these? Sure, but it's not nearly as easy as you're proposing.