Oil tankers only have like 20-30 crew on board, you’re not going to need that many men with AK47s to take over. Navies do patrol piracy hotspots like Somalia, freedom of navigation is kinda important to world trade, but they can’t exactly be everywhere at once.

Which navies though? The reality is that very few countries still have the expeditionary navies capable of doing that. The US Navy (plus Coast Guard) can do it. A few others like China, Japan, India, and France can do it on a limited basis. The remaining countries can maybe send a token frigate on a temporary basis. That's about it. Even the UK, Russia, and Germany no longer have functioning expeditionary navies.