That might work for the current form of the country-specific time zones officially acknowledged by countries that exist in TLD form, but the tz database contains many more current time zones more importantly piles of historical time zones. Representing just the historical forms of currently recognized time zones in DNS would be a mess, without even getting in to those that are not associated with a country that currently exists and has a TLD.

edit: This is a great video explaining a lot of the reasons why time zones are nowhere close to as straightforward as you may think: https://www.youtube.com/watch?v=-5wpm-gesOY

There once was a country named Czechoslovakia.