It must be deliberate. It has been this way, since the new, more elaborate slideshow themes have been introduced, even in iPhoto an Aperture, and the system screensaver. Perhaps the designers are concerned that too long and ill-formatted text will spoil the design.
When I want to see the titles in the screensaver, I have to write the text directly into the photo using the mark-up extension.
We cannot add the titles automatically either in other kinds of Photos projects. When we are creating photo book or a calendar we cannot even open the Info panel easily to copy and paste the title onto the book page. None of the projects can use the metadata in the photos directly.