Two biggest reasons I can think of are:

• chance for malicious software to have been left behind by a previous owner, intentionally or unintentionally. Even if you are going to format the drive once you get it, you still have to plug it into a computer to get to the format stage which has the possibility of installing all sorts of awful intrusive software into your system, even your BIOS/UEFI in some rarer circumstances.

• chance for a failed drive that has been beaten to hell and back. Most used drives are being abandoned because the user wants a bigger drive and has likely used the used drive a ton, causing all sorts of wear and tear on the drive that it won’t be very sustainable and reliable for you, unless you just want some cold storage or have some stuff like gaming where it doesn’t matter if the drive is trashed suddenly and unexpectedly.

Just too much risk for most people to bother.

Same reason why people prefer getting a new car to a used car: you don’t know the care and maintenance, or wear and tear, the previous owner maintained. You could end up saving money… Or you might end up buying a junker.