It's not about the spouse having plenty of time to act - it's about the fact the responsibility is on them to act to end a contract which will be automatically and unilaterally altered in a fundamental way by the transitioning party automatically unless the spouse takes action.
With something as important as a marriage contract, it's not just or acceptable to say "right, your marriage contract is changing and we assume you agree to this change if we don't hear from you by x date". This is a marriage, not a credit card.
The two parties enter into the marriage contract as two distinct legal entities, and transitioning converts one party into a new legal entity with fundamentally different legal features.
This isn't like when a company changes its name and you can easily go on companies house and check its old name to show this company is still on the hook for the same contract - this is more akin to when a company fundamentally charges its nature (e.g. a limited company listing as a plc or other change of majority ownership), which obviously affects the other party to the contract and may well have affected their decision to enter into it in the first place.
A change that alters the fundamental nature of the other party in a business contract, would typically be an automatic breach of the contract in order to protect the other party (and so consent would be required from the other party to enable the contract to continue) because changing the fundamental legal personality of one party necessarily changes the contract, and one party can't change a contract unilaterally.
PPs have cited religious and administrative reasons why the spousal exit clause might be needed, but ultimately it comes down to the fact that a person has a right to consent to marriage and to refuse to marry if they wish. If the person they married doesn't legally exist anymore, they shouldn't be forced into continuing the contract with a different legal entity chosen by the original spouse.
To deem inaction as implying consent for a marriage contract is to force people into a new marriage without their actual consent. Can you imagine if silence implied consent for entry into marriage in the first place? This is no different.