The resistance comes because the ego's thought system is completely proven false when a miracle happens. The ego has absolutely no way to understand it, no way to stop it, no way to interfere with it, and does not know what is happening. This means the ego is going to have a huge resistance to the miracle.
So anyone with an ego left, is going to be resistant to the idea of miracles, because the ego does not believe in cures, or sudden revelations, or instantaneous healing, or defiance of the law of gravity, or any other thing which the miracle demonstrates is possible.
Miracles are expressions of love, therefore expressions of truth. When the truth "happens", it will demonstrate HOW, and the FACT that, all of the ego's so-called "laws" are not true. The miracle bypasses space and time and completely ignores ALL ego laws and just does whatever God wills.
From the ego's perspective, it is frantic to try to deny and block and explain how the miracle did what it did, to try to diminish it and cover its effects so that it will be less obvious that the ego has just been shown to be totally false. Anyone with an ego, which is almost every Course student, is going to therefore have a huge resistance to "real miracles" because they are seen as an attack by the ego. They are the truth, come to demonstrate that the ego's ways are completely illusory and cannot be believed.
There is nothing a miracle cannot do. And that totally blows the ego's mind.
To the ego, miracles should not be possible. There is nothing in the ego's thought system that supports them. When they happen, the ego may go into shock or fear or denial to try to resist it.
If you were in Jesus's presence and some person had died and He told them to come back to life and they did and they stood up and were not dead, you would be like, holy FUCK!
If you were in Jesus's presence and a person that you knew for years who was born blind, suddenly had real eyeballs, and can see perfectly, and there is absolutely nothing in your ego belief system to support how this is possible, you might well run the hell in the other direction!
If you were in Jesus's presence and his body suddenly disappeared in front of your eyes, then reappeared, then disappeared, then reappeared, several times, like someone was flipping a light switch, you'd be like, FUCKING HELL! What the fuck is going on?!!
If you were in Jesus's presence and He suddenly turned the tree you were sitting under into a bunch of flowers and handed them to you, and you took them, and then the flowers turned into a snake which you threw to the ground, and then the snake turned back into a tree, you'd be like, HOLY COW! What the F just happened here?
But in each of these situations, Jesus's demonstration of miracles would have educated you on WHAT IS POSSIBLE, and those possibilities would have taken your awareness far outside the realm of the LIMITATIONS that you are used to believing cannot be transcended. Obviously if Jesus's body is switching on and off like a light switch, it shows you that the body is not really physical. That alone will blow your mind but also you will never again be able to believe completely that bodies are real.
"There is nothing my holiness cannot do." - ACIM
So it's simply that students are anti-miraculous because their ego's are still strongly believing in a state of being where NO MIRACLES happen, a highly unnatural state where there is NO LOVE. That's the ego's entire focus. It doesn't believe in miracles, it doesn't believe it's possible to change one object into another, or that bodies can come back from the dead, or that sickness can suddenly disappear, or that people can grow new eyeballs out of nowhere. It doesn't want anything to do with this ego-law-defying demonstration of truth.
Also given that all resistance to the truth produces sickness and miracles are demonstrations of the truth, people could actually CHOOSE to become sick as a defense against witnessing a miracle. If the miracle were sufficiently outside their mind's ability to accept it as real, they might cling tightly to the body through sickness and pain in order to try to escape from it. Ultimately you could say that all sickness is caused by a refusal to accept that miracles are possible. Or that people get sick because they choose NOT to perform miracles.