Time zones give a rough idea of where the sun is and whether people are more likely to be asleep or not.

Like I know if I schedule a meeting at 8am in New York, that's 5am in California so I'm unlikely get people from California to attend. Or at least it's pretty rude of me to expect people in California to attend

If I schedule a meeting at 12 UTC - that gives no indication of where the sun is in New York or California (or anywhere).

The fact is people tend to do things with the sun is up and sleep when it's down. Time zones are a compromise so your clock is still kind of in sync with the sun while still being able to synchronize with larger areas.

A very useful property of the current system is that calendar days are aligned with solar days and the change of calender date consequently happens when most of the population is either asleep or otherwise not too fussed about the exact date.

UTC-only (or equivalent "global time" proposals) breaks that for everybody not living near wherever the meridian ends up, which causes a lot of weirdness when a single calendar date can suddenly refer to two completely different solar days and two completely different wake cycles.

Additionally, such a proposal would also break all clock time references in fiction, especially when reading works set abroad or in some unspecified (but still Earth-like) setting, but I guess if you don't care about the calendar date weirdness, you wouldn't care about that, either…