Most of the detection platforms referred to in the "billions to develop" are meant to operate as needed, in time of crisis. They're simply not an "always on" affair, and operating costs for airborne detection platforms are expensive. There simply is no need to operate AWACS 24/7 except where a threat of conflict is involved.

Moving to ground radar, many counties simply don't have total coverage. When a transponder works fine the vast majority of the time, and peacetime is the prevailing norm, there isn't a cost justification for it. The prices involved in covering fast stretches of ocean and remote islands with radar coverage is a hard sell.