But as I told you, you have seen me and still you do not believe.
“But” shows a great contrast between what Jesus wants and what they want: He wants to give them heaven, but they only want earth. They ask for a sign that they might believe, but their very asking demonstrates their unbelief. They ask for the bread of heaven, but they must believe to receive. They have seen the signs, but only want more. Their problem is unbelief.
McGarvey gives an excellent illustration. He says that “…Jesus states the hopelessness of the situation. If one refused to believe in the sun when he sees its light, feels its heat and witnesses its life-giving power, by what sign will you demonstrate to him the existence of the sun?” (TFG 385)
They have been eyewitnesses to an incredible miracle. They have seen Jesus do only what Messiah could do and yet they still do not believe.