An especially egregious variation on this theme is evolutionary psychological speculation. I speculate that people do this because in the ancestral environment your audience wouldn't call you out on it if you came up with a fully general explanation of something and asserted it confidently as long as your audience already agreed with your conclusion.
The last point reminded me of speculation from the recent LessWrong article Conspiracy Theories as Agency Fictions:
Before thinking about these points and debating them I strongly recommend you read the full article.