No special reason, other than how would Rails generate the IDs in a predictable fashion? If you are not (or can not) cache the generated html from a template, then the template will be evaluated each time form the server, so Rails would have to have some way of making sure all the IDs it generates are unique across the page - perhaps unique across the whole app depending on what you're testing. Not to mention, you would probably want Rails to use the same IDs for the same elements each time so your tests don't break... So I think you can see it's not one of those no-brainers that it might seem to be.
Why don't you just specify the ID yourself for the elements that need it? It's pretty easy to add your own HTML IDs to all of your link_to statements, or like you said, create your own helper that will do it for you,
Jeff softiesonrails.com
