In case of carboxylic acids the negative charge is on the more electronegative element that is oxygen as shown in the structure in both of the resonating structures. The negative charge on the more electronegative element makes it more stable. And thus it shows acidic character easily.
But in case of phenols the negative charge is shared with the Carbon atom of benzene ring. In most of the resonating structures we can see the negative charge is on carbon atom. As the negative charge is on less electronegative element it makes the molecule less acidic.
Resonating structures of carboxylate ion and Phenoxide ion.
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