I think that's because the practice has many parallels with bad science. Ascribing a metaphor to the author without any evidence is pretty much an untestable, unfalsifiable hypothesis, and therefore has no value.

If you tell me that author X said that the boat is meant to be a metaphor for the protagonist's life, I will be the first to be excited and read the text looking for all the little easter eggs that the author put in, but without any evidence at all it's just your opinion.

This is also exacerbated by the lengths that people go to stretch their theory, which makes it even more obvious that they're grasping at straws in the face of evidence to the contrary.