Are you sure? I recall that the dish was spherical, not parabolic, and had a movable receiver mounted on cables; moving the receiver adjusts the direction from which signals can be received. Indeed, Wikipedia claims that it was steerable [0].

[0] https://en.wikipedia.org/wiki/Arecibo_Observatory

You're kinda both right; the dish itself was fixed, but the system as a whole still had limited steerability because it was (spherical?) instead of parabolic, allowing 20° of the zenith.

This was still a lot more limited than the horizon-to-horizon of the other iconic radio telescope type: https://commons.wikimedia.org/wiki/File:CSIRO_ScienceImage_8...

The competition for giant Arecibo-like telescopes is arrays, and since their cross section goes to zero as you turn them towards the horizon (straight up and they're spaced out, from the horizon they're in a line), they are limited within some reasonable range of the zenith as well.