They are hidden because both of those pages are more easily accessed through other means:
Login is available at the top right of every page on your SwimTopia site and any activity requiring login will present the login anyway.
Registration is better handled by the “Registration Alert” snippet which is only visible when you want it to be—when registration is open and not private.
If you make the registration page visible to navigation you would then have to remember to hide it when registration is closed. Far better to have it hidden from navigation all the time and manage the visibility of the Register Now button through the registration snippet by simply opening and closing registration.