[ColinWright]

You need also to have the code to do the conversion. If you store, in a byte, the years since 1900 (or 1970) then every time you load and/or process a date to be displayed you need to add 1900 (or 1970) and then convert from int to str which takes time (machines were slow) and code.

And the code takes space.

You may think the space is trivial, but I remember working for nearly two weeks on a commercial system to save around 15 bytes on a system, and it was worth it.

Then the culture was embedded ... you worked to save bytes, and didn't do things that would cost more bytes. And the systems worked, and didn't need updated, so they persisted.