What does the word "binding" even mean if it can be unbound by either party for any reason

Many contracts are like this. The only reason why many contracts have these more aggressive clauses is to protect one party from another. You can create a contract with your friend that says he can use your car as long as you say it's ok. What's great about this contract (and marriage) is that it's a way to show the state that this is allowed by both parties. That's the main value of marriage is that it tells the state to treat the 2 of you differently to guard against unforeseen circumstances. A will is a legally binding contract, yet can also be change on a whim, that doesn't make it unenforceable.

If you want you can create a marriage contract that requires a reason for divorce, but generally most parties don't wish to include that in their contract.