[Krutonium]

On any modern phone, your phones user partition is encrypted with a key that is itself encrypted by a key stored in the CPU. When you factory reset, what's happening is basically the key in the CPU is deleted, then re-created. At that point the data on your partition is random noise, so a new encryption key is derived and used to format the partition.

Even better, modern Android then encrypted your personal data with yet another layer based on your password/key/pattern you use to unlock your device. Many layers.

Retrieving that data would be incredibly hard even for a nation state unless the encryption used was deliberately backdoored, and even then once the device TRIM's the space (which it likely does prior to formatting) that data is gone on a hardware level.

(TL;DR Can't move the memory chip to a new device, and even if you backdoor the OS you still need the users password)