[stretchcat]

Those planes have very unusual forms by the standards the public was accustomed to decades ago. In the time since, those planes and ones even more exotic have appeared countless times in popular media, changing the public's perception of what airplanes might look like.

[hindsightbias]

The A-12 was publicly announced in early 1964. It first flew in 1962. The X-3, F-104, B-58 had all flown by the mid-50s. If anything, airplanes have been much more pedestrian as function and form needs were mostly subsonic.

[stretchcat]

As far as I am aware, the A-12 was secret until the 90s, though the existence of the nearly identical in appearance and function YF-12's was indeed made public in the 1960s. But how many in the 60s would have recognized one if they saw it? Public knowledge and common knowledge are not the same thing.

[cookiengineer]

I linked the picture of the radar test especially because of its absurd dimensions. In the bottom left corner you can see a full size military jeep standing next to it.

This thing is _huge_ and honestly, it's very reasonable people thought it was an alien spaceship.