Time zone offsets are whole hour numbers* that are easy to look up, remember and calculate in your head. With your system, you either need to remember to-the-minute offsets to a high number of location *pairs*, or use a computer for everything.

And if you’re wrong by a whole hour with a time zone calculation, it means the distance, on average, is about an hour of sunlight, so when you’re wrong without time zones, you’re wrong by about an hour as well.

* Yes, I know that there are a couple of islands in the Pacific with a time zone on the half hour. No, this is not relevant.