Why syndication feed readers ignore an entry's 'last updated' time
The simplistic reason is that only Atom feeds have a distinction between the date an entry was published and the date it was most recently updated. But as far as I know, even fairly deeply Atom aware syndication feed readers don't try to do anything much with the field; instead they look for changes in the entry's text.
One reason to not use it for much is that the Atom specification is fuzzy about when it gets updated. The specification says that it is only the time of the last significant update, where the publisher gets to decide what is significant.
Another reason is that I believe 'last updated' is inconsistently and erratically implemented in feed generators. Some never update it; some update it at the drop of a hat (somewhat to my regret DWiki is one of the latter, because it has to use the inode ctime as its best proxy for 'has this entry changed?', and inode ctimes can get updated for all sorts of reasons). It's hard to put much weight on something that is so inconsistent in both theory and practice.
But I think that there's a deep reason that feed readers ignore it in favour of looking at the text. The last updated field is something that the server tells them and they would have to take on trust, but updated text is something that they can check themselves. Checking the text is thus much more direct and reliable, and is easier to explain simply to the user: 'this flag on a message means that the text has changed since you last read it'.