The verb wit belongs to special class of verbs in Germanic that while preterit in form are present in meaning. Thus no 3PS in -s (The others survive mainly as auxiliary / modal verbs in English.) I think what's going on here is that there is another verb wot, same meaning, which though dervived from wit is not a preterit-present verb, and therefore declined as normal verbs are in English. It's all pretty complicated, and wot tends to be dialectal in the north of England and Scotland.