The Earth Stargate was first activated in 1945. According to the in-universe timeline, it has been at least a decade since the events of the Stargate movie and at least nine years since regular operations began at Stargate command. Yet it has been kept a secret.
I can understand the politicians and the Pentagon top brass wanting to keep it secret for strategic purposes. However, almost every one of the better characters on the show (Hammond, O'Neill, Wier and even Daniel Jackson at times) subscribe to the idea that "the world is not yet ready".
What does this mean? What kind of catastrophe do they expect? Riots? Collapse of the politico-economic order? Mass hysteria? Worldwide religious cultism?
It goes without saying that a certain part of the population will always respond hysterically or irrationally, but on the whole, it is hard to imagine anything that is so bad that the secret has to be kept for decades (and I'm not even asking about the practicality of keeping such a massive secret!)
Even in-universe, most civilians who were exposed to the secret took it surprisingly well. There were future timelines where the secret was out and nothing bad had happened.
So why is "the world not ready"? When will it be ready?