And remember that fallout 1 and 2 had cactuses as well, plus vault-tec says it will be another 3 centuries since the bombs dropped it will take 300 years for the radiation to fully clear.
You're listening to Vault-Tec? Who constantly lie and conduct cruel experiments on people for "scientific purposes"?
I wouldn't listen to anything Vault-Tec has to say, it's either a lie or a vague explanation of the truth which even then sounds shady.