It was a different time. Being stealth was not seen as possible. Being invisible was highly sought after from the military and people's imaginations alike. Now we have the technology to be virtually invisible on radar and it's not seen the same.
Yeah, but in Wonder Woman's case, you could still see her inside the plane while she was flying it. It was kind of useless. Especially since, as I already said, Wonder Woman can fly.
At least Space Ghost turned invisible along with his ship.