Satellites. better cameras are cheaper than better planes.
Satellite schedules are well known. It doesn’t matter how good your cameras are when everyone knows when they’ll be overhead.
An SR-71 can appear quicker than you can camouflage something you’re attempting to relocate in mid-move.
Maybe Aurora or some other ‘technology sufficiently advanced, to be indistinguishable from magic’ to paraphrase Clarke’s law...
Back then, they could not move satellites easily so a high altitude aircraft that could fly “anywhere” was valuable. Also the side look aspect of the sensors was also needed. Today? I’m old and am sure they have lots of neat toys.