That is true at the end of a divorce, but in the beginning, possession is a very big deal. With a joint account, one partner can legally drain the entire accoun

That is true at the end of a divorce, but in the beginning, possession is a very big deal. With a joint account, one partner can legally drain the entire account to a new personal one, and the other partner has no recourse other than court, which is slow. This is a highly assymetric risk for a lot of couples. If one of the was the sole source of income, they are in a much better position to wait for the court to intervene. This is assuming the money is still there at the end. The offending partner could attempt to hide it. Or they could have genuinely spent it.