There is nothing in your original post about a table. Pages does not export tables to plain text. It doesn't even have built-in features for converting tables to text and the obverse in place — like MS Word or LibreOffice Writer possess.
What you can do. Select all of the table rows and columns in Pages and then copy/paste into a blank plain text (format menu : Make Plain Text) TextEdit document. Or the same paste event into any plain text editor such as BBEdit.
Pages underlying design principal was to be a consumer application without all of the bells and whistles of modern word processing applications like MS Word. Has absolutely nothing to do with quality control on your not being able to export table contents to plain text.
If your table is small, you can select the table contents, copy to the clipboard, insert a Text box, and then paste into that Text box to receive tabular text data.