Because in the astronomy part, we're assuming the GR is correct, and using it to watch astronomical events in a way we couldn't before.

For example, the detected neutron star merger tested that gravitational waves travel at the speed of light, since we were able to also detect the fast gamma ray burst from it. That's testing GR. But it also showed that such fast gamma ray bursts come from the merger of two neutron stars. That's not testing GR.