Time of build not apparent

Perhaps, because those items have dated titles, that must be legally transferred. If an electronic component has a serial number and it’s manufacturer is still in business; they can tell you, within a certain(generally limited) time-frame, when it was built. Because of the model numbers/iterations/serial numbers of my components; I know(within a year) when all of mine were built(just took a little research). Some of my tubes(most of which were made in the Forties, Fifties or Sixties), can be traced back to their locations of origin, as well as month/year, via coding. Regarding tubes; that info can be very important. With most components, I don’t obsess. Apparently; most others haven’t been obsessing either, or- electronics(as well as the vast majority of other consumables) would have been dated(and perhaps, come with Certificates of Origin), for decades.

I understand lower priced equipment, but the 5-digit items should not require research on the part of the buyer.

My Esoteric SACD Player has the month and year manufactured right above the serial number plate.  All AI preamps have the year manufactured built into the serial number.