Why is our household budget a secret?

My wife provides all the income to support all the bills: 2 cars, mortgage, credit cards, utility bills, etc. We are not poor by any means, we live very comfortably. I am a house husband, (kept man?) I have no income, no access to money other than what she allows me access to (usually no more than $300.00), and I always let her know when I am spending anything. I give her all the receipts. I have no problem with this, it's her money. She keeps a private bank account I have no access to.
I have to ask if there is money to grocery shop, buy tires for our vehicles, take animals to the vet, etc.
I have asked at least four times over our marriage of 4-1/2 years to show me the budget: who gets paid when, how much is in savings, what is our budget for a certain item, etc. She refuses to show me anything and gets very defensive that I'm attempting to nose into her business.
I don't understand… I explain to her that if anything happened to her I would be in a horrible situation: no income, no access to money to pay bills, no idea what needs to be paid. She assures me there will be plenty of insurance money to pay everything. I do not understand why I should be put in this situation. Why would our household finances be a secret? This does not give me any sense of security. I can't imagine doing this if the roles were reversed. What's your theory?

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