I've read many many discussions about why manufacturers would list such a pessimistic number on their datasheets over the years and haven't really come any closer to understanding why it would be listed that way, when you can trivially prove how pessimistic it is by repeatedly running badblocks on a dozen of large (20TB+) enterprise drives that will nearly all dutifully accept hundreds of TBs written to and read from with no issues when the URE rate suggests that would result in a dozen UREs on average.
I conjecture, without any specific evidence, that it might be an accurate value with respect to some inherent physical property of the platters themselves that manufactures can and do measure that hasn't improved considerably, but has long been abstracted away by increaed redundancy and error correction at the sector level that result in much more reliable effective performance, but the raw quantity is still used for some internal historical/comparative reason rather than being replaced by the effective value that matters more directly to users.