The original application of dist clustering was dual computers in a single telecom switch. There's not really a need for a security barrier in that case; anyone with access to one computer would be expected to have access to the other.
Additional applications for dist have been explored over the years, but most of them involve clustering servers; where a security barrier isn't necessary; although it might be desirable --- I've used dist clusters where some people had access to only certain types of nodes; bypassing access control using dist clustering was certainly a possibility. Bolting security onto something designed without it often is pretty challenging. Especially if you want to keep all the existing applications working.
Additional applications for dist have been explored over the years, but most of them involve clustering servers; where a security barrier isn't necessary; although it might be desirable --- I've used dist clusters where some people had access to only certain types of nodes; bypassing access control using dist clustering was certainly a possibility. Bolting security onto something designed without it often is pretty challenging. Especially if you want to keep all the existing applications working.