Some people had said with very deep insight: “perception is projection” and if we base any theory in peoples reports of their perception of reality we are bound to get a very hard nut to crack, not because people accounts are unreliable( and that is a factor to consider too), but because every person’s account of a phenomenon will be a reflection of how that person can perceive reality.
Some people have even suggested that the UFO phenomenon have evolved with social development, but I think that they are confusing cause and effect, any description of a complex phenomenon will be a reflection of the times when that occurred, a description of an UFO sightings in the 1800’s will be a reflection of the ideas and expectations of that time, as it is now.
To put a more concrete example, assume that we want to build a theory of birds based on peoples sightings reports of birds. Imagine that for a moment. Some people will say that birds are usually alone, others will say that they fly in groups(flocks), some people even will say that birds are smart because some birds can repeat back some human words, etc.
So any theory build on these premises will be bound to be wrong, all Para-physical theories fall on that category and almost all classical UFO “theories” too.
The “solution” to this is the same solution that scientists had applied for centuries to any similar problem: A systematic controlled study/observation of the phenomenon that will allow with time to decipher regularities and make classification or the construction of a systematic theory based on the observed facts.