Ok yea, that's a good point. Lemme share what I concluded from researching this segment of matthew.
In Matt 16 you see the disciples doubted even after seeing miraculous feeding of thousands. They doubted so much that they forgot to bring the leftover loaves with them when they departed. Then, it was so heavy on their minds that when Christ said to, "watch out for the leaven of the Pharisees" they didn't even think about the strange expression. They just assumed he meant the bread they forgot.
however, Christ then explains by drawing out of them why they doubted. The Pharisees were teaching, correctly, that the bible said in Malachi that Elijah would come first before the messiah. They didn't percieve John the Baptist was Elijah and so they doubted Jesus was the Christ. However, three of them had exceptional faith, and these three are rewarded by being given the very thing the Pharisees were asking for in chapter 16, a sign of his authority and messianic claim in the form of the transfiguration. However only the three saw it, thus the others continued to doubt. and the doubt prevented the expulsion of this demon.
So, considering all this context, Jesus bluntness seems sensible to me. If I was providing all the proof in the world as credentials for a fact, and the people I was showing this to doubted anyway: I'd be frustrated. Especially after it happening many times. So, in my opinion (whatever that's worth), this blunt statement makes sense. It's like, "after everything you've seen you still have no faith in me? Really?"