Failing to put two and two together, to come up with the right answer, is not necessarily a matter of being dense. It is more like a case of not realising the need to do that. Had they compared the previous miraculous feeding of the crowd with this new situation, they might have considered the possibility of another miraculous feeding, but after three solid days of being involved with Jesus’ miraculous healings of so many people, they were likely tired out and ready to see the vast crowd dismissed.
An interesting point in chapter 15 is that the multitude had great faith in Jesus, “and they glorified the God of Israel” (vs. 32). They had chosen to be with Jesus rather than at their employment, they had used up all the provisions they initially brought with them, yet they were thrilled at the spiritual provisions freely given to them. Perhaps Jesus chose to disregard the disciples’ dismissive attitude because the verse also says, “I have compassion on the multitude.”
Jesus had earlier stated (with regard to food and clothes etc) that people who “seek first the kingdom of God and his righteousness” would have all those other things added to them (Matthew 6:33). Jesus chose to demonstrate the truth of that, and so to further bolster the faith of all present, including that of the disciples.
He may also have been testing the disciples, and he may have been giving them an opportunity to put two and two together. The fact that they failed may say more about their own weariness after three solid days of dealing with multitudes than about being ‘dense’.
Jesus did not rebuke them or express exasperation with them. He proceeded to involve them in another miracle, to reinforce their understanding of just who he was. A little later, they had a clash with the Pharisees and Sadducees who wanted “a sign from heaven”. Jesus rebuked them then, privately, told his disciples
“Beware, and take heed of the leaven of the Pharisees and Sadducees”
and they were reasoning in themselves, saying, ‘Because we took no
loaves.’ And Jesus having known, said to them, ‘Why reason ye in
yourselves, ye of little faith, because ye took no loaves? Do ye not
understand, nor remember the five loaves of the five thousand, and how
many hand-baskets ye took up, nor the seven loaves of the four
thousand, and how many baskets ye took up? How do ye not understand
that I did not speak to you of bread – to take heed of the leaven of
the Pharisees and Sadducees?’ Then they understood that he did snot
say to take heed of the leaven of the bread, but of the teaching of
the Pharisees and Sadducees.” (Mat. 16:1-12 YLT)
The disciples were not seeing matters, or hearing Jesus’ words, with faith. They were not applying spiritual understanding to matters. No wonder Jesus told them that they lacked faith! The ‘eyes’ of faith see everything differently, even mundane, every-day matters like eating food. The question is, would we have been any different? Or is our faith in Christ such that we know that those who seek first the kingdom of God and his righteousness will have all necessary ‘other things’ added to them?