It makes a great deal of sense when thinking about the use case: recording a monotinic time in a computing environment with limited resources and extremely unlikely to ever see files from before midnight of new years eve 1970.
With a great deal of existing routines that work with those values and a good number of existing files and storage applications based on the simple version, it makes even mores sense to extend that in an unsigned way with a larger bitfield for any timestamp. Though as recently came up, leap seconds and synchronization could use a few bits to describe which timestamp is in use.
1970 makes sense if you’re picking an epoch and you are in the 70s - everything will happen “after” your start date but picking the actual current date doesn’t seem natural enough.
It’s like picking 2020 as the beginning of an epoch now (of course we’d probably fall back to 2000 as that’s a big round number).
I bet you'd see a lot of Jan 1 1900, 2000, or year 1. Very few would pick 1970.
Other fun non-Jan 1 ideas might be December 26, 1791 (Charles Babbage's birthday) or February 5, 1944, (the date Colossus came online)