Do you have an example of the same quality hard drive being cheaper as an external hard drive? I find that hard to believe, and would presume inferior hard drives are being sold in external cases.
In the comments from the above (you'll find similar in other comments, and on Reddit & YouTube). The Harddrive inside is a WD100EMAZ (WD Red 10TB), which currently retails for £318.68 https://www.amazon.co.uk/Red-10TB-3-5-SATA-5400rpm/dp/B07194...
It's even running the same firmware. It makes no sense, the only logic I see is that customers are willing to pay more for the 'enhanced features'. If you look into this, you'll find no manufacture states the read and write speeds of their USB drives, making it a little harder to compare, so there is a little luck of the draw in what you actually get. Seagate are similar: https://www.reddit.com/r/DataHoarder/comments/8thr34/seagate...
WD My Book 10TB USB 3.0 External - $185.99 (free shipping) https://www.newegg.com/p/N82E16822234346
WD Red 10TB NAS Hard Disk Drive $283.98 (with $3.99 shipping) https://www.newegg.com/p/N82E16822231546
So it's still ~$100 cheaper. Like I said before, this isn't the only example.
I also found this conversation while trying to find out kind of HDD it is:
https://www.reddit.com/r/DataHoarder/comments/8ghb0o/wd_mybo...