Because many of them get a cantrip boost at level 8 that would make it somewhat redundant.Just had this random thought when replying to another thread: since Arcane caster subclasses can get it, why can't Clerics get Extra Attack?
I can't think of a reason against it, other than maybe "giving Extra Attack to Arcane casters subclasses was a mistake", but that ship has long since sailed, lol.