They are hidden because both of those pages are more easily accessed through other means.
For “Login”, that is available at the top 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 active, i.e., open and not private. By making the registration page visible to navigation you have to remember to hide it when registration is closed. Far better to have it hidden from navigation all the time and manage the registration snippet by simply opening and closing registration.
See the Help Center article: Registration Alert Snippet for more about using the snippet.