The oceans are vast. The problem we have is that the area essentially shipping lanes (large traffic), regions where there's some traffic, but massive areas are neither. Nobody is gonna nick your done in antarctic waters. It's hard to get even basic measurements from there, because nobody goes past to even drop a buoy, let alone map the ocean floor.

During the current Ocean Race (massive sailboats, partially riding on hydrofoils) that circles the globe, at least one team was dropping buoys along the way, cause the scientists can't afford to send a ship to do just that. Or find one that just goes there, for any reason. The route goes from Europe past Africa, past Australia, then past south America and back up the Atlantic. The shortest route for that would go through the ice, but there's an exclusion zone to prevent danger from ice. This area is where they dropped them.

There is often nobody within thousands of natural miles. Let alone anyone remotely close enough to even spot a droneship.