The finer the resolution, the more narrow the scanner path is (and in any case satellite scanner paths aren't that wide, also for low-resolution optical scans). The Umbra satellite swath is a tiny 4km wide. And then along-track as well.

The total area a radar satellite (or an optical one) can scan at any particular time is very limited. A satellite, even multiple ones, will only see an airliner if it happens to be in the right spot at the right time.