There isn't really a uniform "they". Some people in Vault 13 wanted to go out. Some wanted to remain isolated, arguing that only doomage awaited them outside. The people in Vault 15 went out and founded Shady Sands. In Vault 8, evidently the prevailing sentiment was that they should go out and rebuild, but take care that such aspirations should not be undermined by ignorant and wasteful outsiders. In Fo2, none of Vault 13 and Vault 15 has its original population.