Radar coverage may not reach remote sites like the 7 Skies wreck. Ships can also turn off their AIS if they want. I heard that some ships sailing through high risk areas (like the Gulf of Eden) will purposely disable their AIS to prevent pirates from tracking them. It's easy for a ship (even big ones) to disappear in the open ocean if they want to.
I think illegal salvaging falls low on the priorities of coast guards and Navies. As it currently stands, the rewards far outweigh the risks for these illegal salvage ships.
Unlike illegal fishing, which Indonesia loses billions of dollars to each year, how much would they lose to illegal salvage operations?
The dive boats from Singapore and Malaysia sure don't spend that much nor visit that often.
One place I know where there's some action to protect the wrecks is in Subic Bay, Phillipines. Over there, the locals will stake out wrecks some nights to chase away salvagers. The local community has recognised the value of the wrecks - tourists coming to dive, spending $ in the local economy. Salvaging the wrecks would wreck (pun intended
) the local diving economy, so they take action to preserve the wrecks, even if it's by themselves.
So back to 7 Skies - who would care enough to patrol the wreck? Divers only visit occasionally and can't patrol the area. It's too far away for local DCs to patrol. The locals (and local government) don't see the wreck bringing them any benefit. The Indon central government surely has more important issues to worry about.
So what's the next best option? Next time someone discovers a nice wreck that's not already public knowledge, keep the GPS coordinates secret! Salvage operations can't salvage what they can't find. I'm sure as hell they don't use underwater sonars to scour the open seas to find wrecks to salvage.