That’s what template entities are for. So you can control the update frequency with triggers.
When relative_time() is used only in UI to show a last-changed/updated - creating a trigger-based templates could be a bit overkill. Actually, rare updating of relative_time() is not a big problem, a user can press F5, just curious about a possibility to alternate a frequency of relative_time().
All 3 functions use the same listener. So changing it would require relative_time to be split out. Also, it’s unlikely core would allow this.