Because it’s what make the time have a meaning. The time when you eat, when you go to sleep or wake up, when you go to work,…
In fact, you’re looking at it the wrong way. The time is localised because that’s how it make sense for people. And that’s how it make sense for physics too. Relativity means each place has its own time.
The question should be why do you want to change this?
Why?
Their value is so that we all can imagine what the sky looks like when we say “seven am”.
But why is that important?
Because it’s what make the time have a meaning. The time when you eat, when you go to sleep or wake up, when you go to work,…
In fact, you’re looking at it the wrong way. The time is localised because that’s how it make sense for people. And that’s how it make sense for physics too. Relativity means each place has its own time.
The question should be why do you want to change this?