Because it makes getting an intuitive sense of what solar time it is somewhere harder.

Can I call my grandma in a different country? Hmm what time is average midnight there. Okay 8 (so far, same thing as looking up a timezone), and it's 18:00 now, so 10 hours after midnight, which is like my 23:00. Needlessly complicated with extra steps for the average person.

Sure, you can say, I'll call you X and that will mean the same thing everywhere, but does not have any information about solar time. And these days, it's automatically converted if you use a calendar (which you should). This is the point of programming, to make the USERS life easier, not the dev. The end is more important than the means, I think we can agree.

Or: what time is it where my grandma is? Okay, cool, I have a sense of what that is immediately after knowing the answer.

There are reasons we do things this way. Working roughly to solar times has more benefits than being able to say a time and it mean the same moment everywhere.

I say we leave things the way they are, works okay.