[p_l]

Nobody expected Unix timestamp to survive so long when it was designed, and norm was to use different format in databases that would care about dates beyond 2038

[bombcar]

And memory was very precious. Why waste bits that would always be zero?

[p_l]

Not just bits - unix timestamp ossified as 32bit value on PDP-11, and handling longer timestamp would involve a lot more code to implement arithmetic on larger than 32bit values, plus also would complicate C language