Teaching is all about crossing the inferential distance between the student's current knowledge and the idea being taught. It's my impression that most people who say "you just have to practice," say as such because they don't know how to cross that gap.

When the specific inferential distance is really really small, people can cross it by doing. This is how things were invented for the first time. And repeating this discovery on your own can be a great feeling that gives you confidence and motivation. So it could be a good teachning technique to do this... as long as you have a sufficiently good model of your student, so you know what exactly is the "really small distance", and if you later check whether the new concept was understood correctly.

So I imagine that while some teachers may really use this as an excuse when they don't know how to teach, I would be charitable and say that a lot of them probably do not have correct understanding of how exactly this works (that very small inferential distances can be crossed easily, but large ones cannot), so they just try copying someone else's style and fail. Actually, sometimes they randomly succeed, because once in a while they have a student who happens to be really close to the new concept, and this prevents them from giving up their wrong ideas about teaching.