Exactly. Sidereal time does get rid of time zones and leap years, but it’s still referenced to a single physical object and relies on a arbitrary choice of start point. So it doesn’t create some perfect cosmic time standard.
The international date line doesn’t help since that’s just 180° offset from Greenwich itself.
The point of standards is that they can be followed by everyone. The AD/BC epoch is fine. The Greenwich meridian is fine. UTC is fine. Changing them would cause so much disruption that it cannot be worth it.