Agreed. I just saw the 8TB Seagate External on sale for $140US. (I paid $10 more

) Find me an internal 8TB for less than $200.
BTW, smartmontools reports the model number as ST8000DM004-2CX188. ST8000DM004 is the model number for the Seagate 8TB Barracuda. I think if I shucked that drive (which I do not plan to do) that's what I'd find inside. (And Amazon lists it at $199.48US so I should have said "much less than $200US

)
The 8TB WD External (WDBBGB0080HBK-NESN) lists at $160US whereas the 8TB internal (WD80EFAX) is $260.
I would expect the externals (with a similar internal) would be more expensive due to the need for the housing, SATA<->USB adapter, cable and wall wort. In addition they are more likely to get jostled around and it seems like that would lead to more warranty claims. My expectations would be wrong.
One thing to be cautious about is that some external drives use SMR recording technology to squeeze more on the drive. Apparently this really slows them down. (I've checked copies on my external and they run ~100MB/s which seems reasonable to me.)
I've also heard that USB is broken in that the protocol allows the device to report that data have been written when it is still in the buffer. That is a Bad Thing™ if a write operation is interrupted for any reason (crash, power failure, trip over cable ...)