Human beings have forgotten all kinds of things. We only recently rediscovered how the Romans made a cement that became stronger when in contact with seawater: "Ancient Romans built concrete sea walls that have withstood pounding ocean waves for more than 2,000 years. Now, an international team has discovered a clue to the concrete’s longevity: a rare mineral forms during chemical reactions between the concrete and seawater that strengthen the material."
Technology can't exist in a vacuum; it has to be supported by an education system which produces people who have a knowledge capable of understanding how that technology works, and it has to be underpinned by all kinds of other theoretical and practical technology. E.g., it's no use knowing how to make batteries in a society that hasn't discovered metal working. Presumably the original Shinra lived long enough to father a child on The Planet, but we don't need to assume that any other crew members survived, or that he survived the impact with all his memories and faculties intact, or that he lived long enough to instruct his child or children in advanced mako-powered airship mechanics, or that the society into which he'd fallen was capable of producing the raw materials and parts he'd need if he was going to build a reactor.
It's very arrogant of us to assume that we could understand and use a piece of alien technology if we found one.
Thank you. Precisely. There's still debate on the recipe of Greek Fire, for Athena's sake.
Hell, a descendant of Shinra's knowing that their ancestor had once tried developing a means of repurposing energy siphoned out of planets is no more useful knowledge than when Shinra initially speculated in X-2 that "With a little work, we could probably extract the energy in a usable form" or when Rin said "We are researching ways to extract the vast energy that sleeps in Spira."
It was an
idea with no known practical means of implementing it. How long was mankind working on "flying machines"? We didn't go straight from da Vinci's designs to Orville and Wilbur Wright in a single generation, and theorists had been trying to work it out for centuries prior to da Vinci as well.
Vegnagun could be used to extract mako energy. A system of repurposing it was still required, though.
To revisit Licorice's battery example, it does one little good to suggest using one object to power another without knowledge of processes that will generate energy, and it still does only so much good to understand that there's an electrochemical reaction between zinc and magnonese dioxide if you haven't developed an architecture in which this reaction can be produced, contained and sustained, with its resultant energies then conducted.
We didn't go straight from "I need something better than this oil lantern" to Duracell-powered flashlights. Thankfully, we also didn't suffer any civilization-resetting calamities between the development of alkaline batteries and Maglite. FFVII's world, on the other hand, did suffer such a calamity.
By any measure of canonicity, the civilizations of Cetra and non-Cetra alike -- magical technology and the more traditional sort -- came under attack and the present-day descendants of the survivors demonstrate little knowledge of their species' history.
Hell, apparently no one even remembers the names of the towns that became Midgar over the few decades prior to the game. Why would we think they had done any better than that with retaining the entire history of civilization?