What would make you say that? I know William Gibson wrote The Difference Engine, often seen as the start of Steampunk, but the book seemed more a work in alternate science-fiction, much like the works of Harry Turtledove. And if we count the works of people like Jules Verne and HG Wells, who postulated advanced technology based on the technology of their time, their fiction could retroactively be seen as “steampunk”.
That, to me, is one of the hallmarks of steampunk: as a form of retrofuturism. Different generations and time periods will have their own imaginings of the future, like how the first Alien movie had CRT monitors in the far future, something that Alien Isolation carried over. Thus, Steampunk’s appeal, as with Raygun Gothic, is as a future that “could have been”.