Why do people care about absolute certainty?

...in the sure and certain hope of the resurrection.....

You make a valuable point. Philosophy threads in particular seem to attract rhetoric rather than informed response. I plead guilty to overkill against those who put tend to put rhetoric before reference to acknowledged informed sources.
The phrase "absolute certainty" is in common use and extensive literature exists on discourse analysis which throws light on the contextual difference between semantics and logic associated with such a phrase. We can even imagine religious waverers "losing sleep" over this type of emphatic phrase since a variant of it is contained in the Christian burial ceremony.

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...in the sure and certain hope of the resurrection.....

Don't loose sleep over it. Frank was looking for a spanking and you provided it to him...

For much of the history of linguistics and the positivist philosophy of language, language was viewed primarily as a way of making factual assertions, and the other uses of language tended to be ignored, as Austin states at the beginning of Lecture 1, "It was for too long the assumption of philosophers that the business of a 'statement' can only be to 'describe' some state of affairs, or to 'state some fact', which it must do either truly or falsely." He was one of the first people who made a systematic account for the use of language. Latter Wittgenstein came up with the idea of "don't ask for the meaning, ask for the use." showing language as a new vehicle for social activity. Speech act theory hails from Wittgenstein’s philosophical theories. Wittgenstein believed meaning derives from pragmatic tradition, demonstrating the importance of how language is used to accomplish objectives within specific situations. By following rules to accomplish a goal, communication becomes a set of language games.

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For much of the history of linguistics and the positivist philosophy of language, language was viewed primarily as a way of making factual assertions, and the other uses of language tended to be ignored, as Austin states at the beginning of Lecture 1, "It was for too long the assumption of philosophers that the business of a 'statement' can only be to 'describe' some state of affairs, or to 'state some fact', which it must do either truly or falsely." He was one of the first people who made a systematic account for the use of language. Latter Wittgenstein came up with the idea of "don't ask for the meaning, ask for the use." showing language as a new vehicle for social activity. Speech act theory hails from Wittgenstein’s philosophical theories. Wittgenstein believed meaning derives from pragmatic tradition, demonstrating the importance of how language is used to accomplish objectives within specific situations. By following rules to accomplish a goal, communication becomes a set of language games.

“Not to be absolutely certain is, I think, one of the essential things in rationality.”

Your tedious game suggests that I find an "authority" daft enough to indulge in ridiculous semantic cheese paring with respect the unlikely event of a non-contextual juxtaposition of "certainty" versus "absolute certainty". Sorry, but nobody seems to be stupid enough to play that game, because thinking people automatically understand the nuances of the emphatic term "absolutely" as used in context, and don't waste their time with logical pedantry. The best I can do is this:

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“Not to be absolutely certain is, I think, one of the essential things in rationality.”

Bertrand Russell. (Pioneer in Logic and Author of "A History of Western Philosophy").

A stupid man's report of what a clever man says is never accurate, because he unconsciously translates what he hears into something that he can understand.

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A stupid man's report of what a clever man says is never accurate, because he unconsciously translates what he hears into something that he can understand.

Russell, Bertrand. A History of Western Philosophy. 1945.