Uh, actually, astronomers with telescopes have observed phases in Mercury and Venus, the planets closer to the Sun than Earth is.
We don’t really see phases of planets further away from us from Earth very well, because we’re basically always seeing the side that the Sun is illuminating.
Probes such as the Voyagers, Cassini, Galileo, Juno, and New Horizons have taken pictures from the “side” or “behind” planets that show phases, but of course because that blows up your argument you’ve already dismissed them as “cgi pictures”