Lines (chains, as per your article) have a "lot less area" than rectangles do, though. In other words, it's way, way easier to answer "what has crossed this line, at what time?" than "is there a needle in this haystack as large as Europe?", especially when the rectangle in question is basically of zero interest 99.9999% of the time, and you're tying up resources that could be pointed at China, North Korea, Russia, Iran, San Marino or other more 'interesting' places.

Furthermore, as your article mentions, they decommissioned some of that because it was too expensive and didn't serve much purpose with the end of the cold war. Even those guys' budgets are not unlimited.