Priests -and I'm keeping this to AD&D priests because I don't want to spark a full scale theological debate or argument, are considered to be focuses for their gods power.
Obviously the more powerful the cleric the greater the power there is no reason why turn undead would stop increasing in potency.
Imho mage spells are limited in power because mortals can only control a finite amount of magical energy. A high level cleric is going to be looked after by his god and will not be put under the same pressure.
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