Stable-Tec Stables weren't just built to save the population in the event of a megaspell attack. Unbeknownst to the majority of Equestria at the time, each Stable was constructed with a specific goal in mind, a particular focus to study.
Built just outside the city of Chicoltgo, Stable 1 was advertised to be able to house one thousand individuals safe from annihilation. In reality, Stable 1 was designed to be occupied by as its name would otherwise suggest: just one.