The widely accepted explanation is that what would become the Galactic Empire would take about two decades to build something as massive and complicated as the Death Star. No small undertaking.
As a 2016 Inverse article explains, this is supported by the events of the prequels of the late 1990s and early 1990s. In short, Count Dooku (Christopher Lee) is shown to be in possession of the holographic schematics for the Death Star in “Attack of the Clones” and by the end of “Revenge of the Sith” a few years later, the ship’s frame is shown to be in Building. 19 years later, Jyn Erso (Felicity Jones) and her gang of rebels steal the plans for the completed Death Star in Rogue One.
The article also warms up a bit of the back-and-forth between fans as to whether the math holds. In the end it seems so. This is particularly prescient given the mind-blowing post-credits scene at the end of the season 1 finale of Andor. Here we are shown that the parts Cassian (Diego Luna) and his fellow captives built during his short time on Narkina 5 are intended for a near-complete but far from operational Death Star. Given that Andor is set to take place five years before Rogue One, the math works again.