Why was a complete test flight of Hughes Hercules H-4 (Spruce Goose) never undertaken?
By the time the government had pulled the plug on the program. Even that hop got Hughes into trouble because they had been expressly forbidden from taking off (he got off on a technicality, claiming it'd been unintentional, unexpected behaviour from an experimental machine, etc.).

If it had been anyone except Hughes I'm pretty sure the H4 would have been destroyed rather than preserved as a museum piece, the US government was that intent on killing the project there and then. They had a lot of air bases with long runways to find a purpose for, and long range air travel was an excellent purpose. They also wanted to find customers for the thousands of surplus transport aircraft (and bombers to be converted to that role). So getting rid of the traditional way to fly intercontinental, the flying boat, was important.

Were people curious? No doubt. But they also were budget conscious, the H4 had no more military application so there was no money coming out of the Pentagon. It had no civilian application, as for large payload ships are cheaper and for small, fast payloads it was too large. So even if there had not been active hostility towards the flying boat concept at the time from people with the power to release or deny budgets, there would have been little point in implementing an expensive flight test program (which usually requires multiple vehicles...) for an aircraft for which it was highly unlikely there would ever be any peacetime orders.

Of course 50 years later the idea was kinda revived with a short period of interest in Wing In Ground effect vehicles to fly troops and supplies over the feared Soviet submarine fleet in the Atlantic. But that didn't start up until near the demise of that threat in the 1990s and nothing ever made it past concept drawings (except paradoxically a few Soviet craft in the Caspian and Black Sea regions, the Ekranoplans).

I've little doubt the H4 would have worked as advertised, seeing it it looks sound in design, basically a scaled up B314. But there was just no market for it. It was an example of too big, too late.