Why does year 0 become year 2000?
Because that is how it was designed and implemented. See here


  If BC was not specified, and if the year field was two digits in length, then adjust the year to four digits. If the field is less than 70, then add 2000, otherwise add 1900.


(although two digits should probably be "one or two")

People often use the last digits of a recent past or near future year to represent the year, dropping the 19 or the 20.  This heuristic was designed to accommodate that raw fact.

That the year 0 doesn't exist has nothing to do with it.  The years 1, 2, and 69 do exist, and yet the heuristic applies to that as well.