Why is my Find tag from selection option always greyed out (under Options in the Tags panel)?

When in the Content panel I can click on a bit of text (which is mostly all vertical! is this normal??), and the corresponding text is eventually highlighted in the document to the right . . . why isn't it this way for the Tags panel?? I suppose I shouldn't even ask Why questions like that here, but my subject line is a valid question . . . I'd really like to use this feature but it's always greyed out. Thanks for any pointers.