The resurrection is the one sign for the real question asked by the Pharisees which Jesus knew and could be imagined as: we heard you have been doing signs but that is not enough and we won't believe if we dont see by ourselves. show us something that really confirms you are the Messiah or we wont accept that
Ultimately, considering the way the Lord answered, they shall be seeking to prove the opposite of that.
Read it again: "Then"Then some of the scribes and Pharisees answered him, saying, “Teacher, we wish to see a sign from you.” But he answered them, “An evil and adulterous generation seeks for a sign, but no sign will be given to it except the sign of the prophet Jonah" - Matthew 12:38-39 (ESV)
They were seeking to know if Jesus is the Messiah and the resurrection is the ultimate sign. Consider that prophets also made signs.
Ellicott's Commentary for English Readers: "There shall no sign be given to it.—The words seem at first to place our Lord’s miracles of healing outside the category of signs, and yet it was to these that He referred the messengers of the Baptist as proof that the Christ had indeed come (Matthew 11:5). They must, however, be interpreted by the context. One sign and only one, such as they demanded, should be given to those for whom the other notes of Messiahship were insufficient, and that should be the sign of the prophet Jonas." - from https://biblehub.com/commentaries/matthew/12-39.htm