Why? I hear and see this - it's almost a defacto "standard" for audit scheduling - and yet I can find no reason for it...
Simple reason: to keep things simple, and ensure no process gets ignored - and, like I say, it's just a baseline that can be modified in light of data.
As an analogy, take flossing your teeth:
The "defacto standard" might be once-a-day. Why? Maybe it's not necessary to floss everyday...
You could certainly consider day-by-day such factors as "what did I eat today?", "when was the last time I flossed?", "what were the results of my last dental checkup?", etc., and then modify your flossing schedule accordingly... This way, some days you might floss several times, sometimes you might go days without flossing.
Is this approach better for your oral hygiene? Is it easier for your life (management processes)?
In my view, it's generally simpler to just adopt a baseline schedule - with allowances for deviation - to ensure there is a minimum assured level of maintenance.