Any reason why not in the caption bar?
Design decision.
The file window caption bar wraps longer folder/category/collection names into a second row as needed, and this would overwrite the comment. We discussed this during the Alpha test phase long ago and I've found a corresponding comment in the source code. I decided to not use the overflow space in the caption bar and to display the comment instead in the fly-over tooltip (and now also in the thumbnail tooltip).
This can be changed with some effort and additional logic. If there comes up an FR with sufficient Likes, I'll look into this for a later release. I doubt that the feature to add descriptions to folders and categories is used by many users.
As an idea for the OP: What I do: I store a simple
HTML file in the folder (often) or category (rarely). I could also use a plain text file but I like to have some formatting abilities.
I name this "_description.html" so it sorts to the top using my default sort profile. The Quick Preview panel displays HTML files using Internet Explorer and this means I cannot only store text but even format it if so desired.
I have a small IMatch script which creates these HTML files for me. I then just open them in an editor and write whatever I want to remember about the photos in the folder. Since it is HTML, I can also link to other files, web sites etc.
I don't need to mirror the per-folder or per-category description in metadata. I use the description tag to store per-file descriptions, not information about the folder (irrelevant) or the category (a file can be in many categories, what if all have descriptions)?
But if I would like to copy the folder/categroy description to the file metadata, I would just select all files, then select the text in the Quick View panel, and paste it into the XMP tag I want to fill in the Metadata Panel. Done.

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