Your question is, essentially, what are the rules for when we consider something has been invented? I touched on this in a related post recently. There are some very big assumptions tied up in the simple phrase "the first airplane", "the first automobile", "the first steamboat", because in all three of those inventions, there were early fuel-powered devices created that were intended to carry a human into the air, transport a human along a road or push a human through the water, and a substantial time interval between them and later devices that were more successful. For example, there were powered road vehicles before 1900. Yet they remained somewhat novel until Henry Ford started making the Model T. Likewise, there was a steamboat, built by John Fitch, in 1787 . Yet steamboats were something of an oddity until Robert Fulton made his a financial success 20 years later. After Ford, there was an unbroken growth of automobile manufacture and use. After Fulton, the same happened with steamboats. Therefore, a lot of people tend to think that Fulton invented "the" steamboat and Ford invented "the" automobile. And in a sense, they're right...sort of in the same sense that Columbus "discovered" America- after Columbus, America was known. After Fulton and Ford, steamboats and automobiles were known.

In the case of the Wright brothers, others had created things with wings. Many designers, like Lilienthal, had managed to build something with wings that could lift a human into the air. It's clearer to say that they were the inventors of controlled flight, because that was the greatest problem remaining. Because of that, after the Wrights, airplanes could happen, so people can commonly think of them as having invented "the" airplane.

Of course the Wrights did not start from scratch. They benefitted from previous research, previous failures, read everything they could find out about previous airplanes, including Lilienthal's. They also were able to benefit from advances in gasoline engine design that could give them a reliable engine, and the advances in aluminum smelting that made it less of a precious metal and could make them a lighter engine. And they just had some luck, as well: Lilienthal himself seems to have been working his way towards some sort of warpable-wing , and if he had not died in a test flight he might have gone on to get the credit for at least the first working glider.

The Wrights also were quite aware not only of the previous work of other aviators but how their own airplane might be received. People like Langley and Santos-Dumont had been able to get things briefly into the air and raise hopes. The Wrights wanted to show a clear difference between those designs and their own. They spent quite a long time working out problems in their airplane before they started making public demonstrations of it because they wanted to show something that didn't just hop and come down, but stayed up.

But that does not mean that others had not previously done a lot of work in human flight, and some of those people should be better known. And not necessarily the ones with flying machines. Gustav Eiffel not only built that tower in Paris but created a very advanced wind tunnel for aerodynamic experiments- yet despite creating that immensely useful tool, his name is not usually associated with flight.