I am pretty sure this isn't the case if you report the phone stolen. Your provider will have the IMEI number and can brick it. There are probably ways around this - one that I've heard is that this bricking is regional. If you sell the stolen phone to someone who is going to ship it to Africa, say, and resell it, it would work fine there.

I should say that this info is a decade old, but I knew someone back then who would pay for phones, no questions asked. Also vehicles, even large commercial ones. The containers were going to the Gambia, although I'm sure other people were shipping stuff to other countries and continents. I don't know if bricking is still regional but I've not heard that it's changed.

Another possibility is that thieves are trying to literally snatch a phone out of the hand of someone who is using it, while it's still unlocked. Many of us do banking etc on our phones, and have other login credentials, so perhaps if they get the phone while it's unlocked they can do something with this.