Where do "get this idea?" You can easily search google for "sea piracy" to find out that pirates can successfully hijack ships.

Merchant ships on the high seas are not the responsibility of any particular nation, nor are most nation-states particularly well equipped to keep track of vessels in their territorial waters, especially if navigation of those waters is complicated (see Indonesia). The idea that countries would dispatch jets (many countries don't even have an airforce) every time a transponder malfunctioned is...laughable? Countries simply don't keep track of vessel locations that way.

As for what one does with hijacked ships, tankers are the best target: you simply pump the load to non-suspect vessels and sail them to countries with corrupt or gullible customs officials.

The impact of piracy on the international oil trade is small, but there's no reason to expect it to get smaller as the value of a tanker load increases.