... and yet, they exist, are cheap, and widely available.

  Burner phones are available at all major retail outlets, including Best Buy, Target and Walmart. They're also often available at convenience stores like 7-Eleven, local supermarkets, gas stations and retail phone outlets like Cricket and Metro.

  You can get a burner phone with cash, and it should cost between $10 and $50, although it may cost more if you get more minutes and data.

~ https://www.cnet.com/tech/mobile/what-is-a-burner-phone/

Doesn't have to be a whole phone either, can just be an over the counter burner SIM.

You probably have seen someone use a burner phone, it's not like a giant red arrow appears over users head with "burner phone" in blinking letters.

They want to get rid of burner phones so spies and terrorists won't be able to use them anonymously anymore. This is just like when the FBI got the unlock code for iPhones to read a terrorist's locked iPhone; now they can unlock any iPhone.