A contract requires agreement of all parties? Since when? It requires various DEGREES of agreement, sure, but not everyone needs to agree on everything. Governments which rule through fear rely on the citizens agreeing that they risk too much, with too little potential reward, to try to change the system, leave the country, or take up arms against it. That's agreement.
"Don't do this, or you will be killed" provides a very good incentive for a lot of people to not do something, but put enough guns to people's heads, and ruling by fear hasn't been shown to be very effective for long periods of time. Tyrants more often than not die bloody deaths.
It's hard to keep everyone peaceful when motivated by nothing but fear. Although yes, if someone puts a gun to my head and says "give me all your money", it's a rather strong contract between me and the assailant. One I'm very seldom willing to risk my life in order to break. Some people are. Those are "heroes", or "brave individuals", or "world kickboxing champions". I am not.
Neither are most people in repressed countries. But the government is still functioning most certainly due to collective agreement.
Offer and acceptance has been the requirement of a contract for at least thousands of years. But I am glad you believe that, it means our contract is in effect. I am thankful for your implicit agreement to the contract and terms that I never presented to you and that I can change at my whim (just like a government "social contract"). I'll be expecting $1000 from you at the end of each month.
Why do you believe acceptance is accomplished by words, rather than actions? If someone holds a gun to your head, damn right that's a contract. "Give me money, or I put a bullet in your head". You can either accept the terms, ie, give money, or reject the terms, and either attempt to evade the consequences or test how strong the leverage was.
The robber presents the terms when they corner a person and have their life in jeopardy. The robbed accept the terms under duress.
You, on the other hand, have no such leverage. You aren't in a bargaining position by which you can extort monthly instalments of 1000$. (Nor are your common robbers on the street, for that matter. Giving up what's in my wallet, compared to monthly 1000$ payments, require very different sorts of negotiations.)
Most humans don't necessarily need to resort to robbing random people on the street, thankfully... and we have instilled a collective agreement that such actions are to be subject to penalties such as prison... some countries more robustly than others.