1. I don't want you to do that because of x,y,z.
2. I saw some other people try that, and they failed.
3. Someone else doesn't want you to do that.
4. I tried it, and I failed at it and my ego is tiny.
5. Rule or Law X states you shouldn't do that.
Can't is such an abused word, better to not use it at all to make yourself clear.
When you understand the laws of physics, anything is possible.
Those people just inherited preconceptions from their environment (co-workers, bosses, common knowledge...). Their brain never even tried to wonder if a new approach was possible.
I think this article describes the situation well. At first I was annoyed with them, but then I came to realize that the reason they were saying this was because they didn't understand what I was getting into, and from their perspective it really was impossible.
I have come to appreciate that they are not out to rain on my parade for the sake of being mean, but because they actually do care about me. I suppose that as with all criticism, discernment is needed.