The '80s in particular were a time of greater trust in government and greater mass conformity in popular culture. The media by and large did not challenge these absurd estimates, nor did schools, nor did, largely, the public. At the time NASA was near the peak of its reputation. It had achieved the moon landing, it was the middle of the Cold War were NASA's achievements were an important battleground, and so on. Overall the tendency was to simply defer to NASA's judgment.
There was also a bit of self-delusion going on. The Shuttle system was, by design, the backbone of spaceflight (manned and unmanned) in the US at the time. On the one hand you could believe that the Shuttle system was a modern miracle, fully capable of achieving (or nearly so) its design promises of cheap and ubiquitous spaceflight, ushering in a new space age, including the launch and assembly of a next generation space station in the near future, and possibly including the realization of manned missions to Mars within the next decade or two. The competing view, that the Shuttle was a risky launch system that could never achieve its design promises even within an order of magnitude, was a vastly depressing (though in retrospect realistic) one. Holding that view meant that we would have to go back to the drawing board and spend maybe another decade building a new launch system that would reset us back to the way things were in the 1960s, and then we'd have to slowly crawl our way toward incremental progress. That was a very difficult truth to accept, ironically more difficult the more you were invested in space exploration.
Sometimes reality is a tough pill to swallow.