Well, one factor is the nature of the procurement process, particularly back in the 1940s and 1950s when there were a lot more aerospace contractors than there are now. Many of these companies were trying to nab government contracts, so they worked up various ideas they had and presented them, but were not able to garner enough interest to go forwards. Other times, some part of the relevant agency has an idea or concept, and works up the idea to sell it to other parts of the agency (eg., the space people in the Air Force trying to sell space-based weapons to the rest of the Air Force). In both cases, there's an incentive to develop concepts so that you can explain your idea more easily and hopefully gain some support.
And usually there is some intention of building whatever it is, it's just that there's no interest from the Army (Air Force, NASA...), or budgets are lower than expected, or there's some kind of technological breakthrough that makes the entire concept obsolete (this doomed a lot of the cooler piston-powered planes from World War II, or hypersonic "air"craft in the 1950s), or changing economic circumstances (eg., tipjets tend to consume a lot of fuel, so when oil prices started rising in the 1960s and 1970s, tipjet aircraft became much more expensive for their capabilities than a more conventional helicopter), or other unexpected changes or circumstances which mean that there's no ability to further develop or pursue the concept.