I mean, it doesn’t mean we couldn’t, just that it was not worth revealing our ability to do so over one airliner crash where no one was likely to survive. They didn’t directly act on intelligence in enigma for the same reason either if memory serves.
Tin foil hats are itchy…
If your argument is "MH370", it's a weak one. No one has radar coverage in the southern Indian Ocean because no one needs it.
There was an article about that very topic on HN just recently. It was not even remotely over US airspace, and the pilot had evidently threaded the needle through the radar coverage of several nations.
What if its not actually feasible to do that? We have lot's of sensors and cameras everywhere, those would have been redundant if total surveillance was possible.
Most of the ocean is unmonitored, too. Remember MH370? We lost it in the air and we couldn't find it in the ocean. :-)