A timezone is a constant (barring DST shenanigans) offset, which works for all the hours of the day. I can look at my watch here in Germany and I know that it’s 8:15 in New York right now. So I know that it’s still early in the day for my buddy Jeff.
In the same-time everywhere logic, I would need to remember specific times, like “people in New York usually start working at 15:00 and stop at 24:00”, which is just plain inefficient.