For the same reasons that you have been tired after 4 sectors on the 767 on day 5 of a domestic pattern on min rest.
See this is what I find is interesting. I know the tiredness you speak of. I understand the jet lag, long tours of duty, as well as the accumulated tiredness from 4-5 days of early starts and long days. I've experienced it all.
I'm not sure it's ever made me require an ILS on a CAVOK day though. My logic being that if I can't function and do a visual approach on a day such as you describe then I probably shouldn't be there to start off with.
I have required 16 (or 34) in Melbourne at the end of a long day when 27 the duty runway but that was more about it being northerly with a variable tailwind and potential overshoot shear and a pretty heavy aeroplane rather than my tiredness levels alone.
Anyway, each to their own. Like I said in my last post. If you're requiring an ILS on a CAVOK day then the rest of us don't know what baggage you're carrying (metaphorically) but it won't stop us from saying 'WTF?' to each other across the flight deck when we hear it!
Require away to your heart's content. Now that I'm an A380 driver it's about a beer a minute for any delay!
(I note the edit as I was typing KZ Kiwi. I'll leave my comments as is. I understand the point you're trying to make. I'm not sure whether I'm just missing the nuance or perhaps just look at it a bit differently).