Hidden from who? And accessible by who?
Mainly because of geo-restrictions.

eg. UK players unable to access PokerStars new private tables at Playtech.



But also curiosity examples;

eg. when Winner limited access to their own private tables (weren't visible in casino lobby).

or, like when yourself LD were unable to get onto Betfair's mustard blackjack table.

or a very longtime ago on a thread (recently deleted ) regarding Bet365's first roulette table at Playtech. Again, all Bet365 players weren't allowed access to it (wasn't visible in casino lobby).