Same reason why people are generally much more civil in real life than online - the risk of being punched in the face keeps everyone in check and assigns a real cost to assholic behavior.
Does a marriage contract bear the risk of injury? How about a business merger?
This is weird logic that belies a desire for vengeance, not justice.
Keep in mind that the potential for violence doesn't mean violence has to occur - every party in the contract is still free to not force the other side to resort to violence (which usually happens out of desperation).
The possibility of someone reacting unreasonably however keeps everyone more honest and assigns an actual cost to pushing people to a situation where they will resort to violence out of desperation.
I'm talking about the repo goon having to risk that the car leasee he's repossessing from being at their breaking point (because that car is the only way they can keep their job and avoid losing everything) and/or mentally unstable and pulling out a gun or a crowbar and thus pricing this risk into their services (or outright declining the job if it feels too sketchy) which ultimately trickles back up to the lender and encourage them to let go or be more lenient with this particular case (something they'd have no reason to do if they could turn the car off remotely).
I don’t think it’s reasonable to expect a threat of violence from the enforcement of a contract. There’s a reason why we tend to give the monopoly of (non-self-defense) violence to the government.