Allen was from a small town and high school with no football pedigree. He was discovered, almost by luck, by a junior college coach, then after a year at that school and still almost totally ignored, he went to Wyoming. At Wyoming, Allen's receivers dropped almost as many passes as they caught.
At no time did Allen receive any indepth coaching. Instead, all along the way, it was his physical talent that his coaches relied upon. Tom Brady was a sought after talent (even drafted by the Montreal Expos), but at Michigan he had difficulty winning the starting QB job. Jim Kelly left Penn State after Joe Paterno thought he was more suited to play linebacker. Proving that even major colleges can't recognize Hall of Fame talent when it's standing, in uniform, right before them.
Allen never had any elite coaching, or attempts to alter his mechanics, until he reached the Bills. If he had, he never would have made it to the 7th pick in the 2018 draft. Even then, it wasn't until after his 2nd year when he had some intense coaching and analysis of his mechanics, that he blossomed. It also shut-up many experts who had declared that accuracy was the one QB trait that was almost impossible to improve.
Allen is an anomaly that practically fell into the Bills laps. He ended up in the perfect situation, with the perfect organization. Compare the Bills to the dumpster fires in Arizona, Miami, Carolina, and Denver. Places where they've had, or have, talent and squandered and mishandled players. Miami has been hot and cold on Tua since they drafted him. Carolina has both Darnold and Baker and is mishandling everything. Arizona wasted Rosen and Murray might be happy to walk away from the team. Would any of those organizations, had they drafted Allen, had the patience for him to develop?