[dogma1138]

Not sure if it's that easy you'll need over the horizon radars to have coverage over the ocean and it's not like they are cheap to build or easy to operate.

You'll also need to integrate that system across borders and jurisdiction which isn't easy politically.

It will also not surprise me if many actors want to have certain level of privacy as far as international airspace goes.

But the biggest factor at the end is cost compared to the relative costs of S&R missions if you did had additional tracking data. In the past 25 years only 3 scheduled passenger flights were not found including MH370 there were quite a few other private, chartered, and cargo flights that went missing mostly in tricky areas to search for in the first place like the Himalayas and the Indian Ocean.

It's doubtful that better radar coverage could even help with such cases even if you build a network of over the horizon radars which will scan for aircraft at various altitudes e.g. 2000-30000 ft. This will be very expensive and when you have a small aircraft that crashes onto a mountain side in an area where S&R can't get too or crashes over a 5KM deep ocean they might very well not help that much.