Because for us students it seems that there is no structure for it. We get the impression that there is a lot of assumption that one might do in order to prove by induction and there is no formula for it. Every question is different and brings in a new factor or a new way of creating the base case, or splitting the equation to be proved, or inserting a random number at the end that makes the equation still valid, etc. There is always something that we haven't seen in previous proofs that makes it really challenging for us to just come up with this new "trick" for every new thing we try to prove. And it is frustrating. There seems to be no method to learn it. We see other people doing it and think it is a natural talent that we don't have and no matter how much we practice we will never learn. When I see someone doing a proof by induction, during the the process I am often like: "Where did you get that from?!?!". And the answer is always like "Oh, that is simple, you just do this, this, and that... tadah!!" And what the person does is always something I have never seen before. Always. How did he come up with that?!! So, yeah, that is why I struggle with induction.