The reason is that if a wife chooses to stay at home and raise children, giving up progressing her career, the government has an interest in protecting the wife financially, should the husband be viloent, adulterous, run away with another girl or just a deadbeat in general.
Err...I'm not really sure THAT is how I would put it.
Anywhooos...my take is that marriage is a commitment for life. I understand that people and circumstances change, and therefore divorce happens, etc, but the reason things are owned jointly is that you have joined your life. It's obviously going to be a little tricky to 'unjoin', but then again, no-one forced you to get married. And any difficulty working out who gets the car is going to pale into insignificance when you end up dealing with young 'uns. Who owns them?