but it seems to me that it really puts a big barrier in their relationship to continually have to sign contracts/agreements/documents to remind them of their status and shared responsibilities (kids, mortgage, insurance, etc) year after year.
Those generally still require both signatures even when married. We both had to sign the paperwork to register our daughter for school. Mortgages require both to sign (assuming you're both on the loan). Home purchases require both signatures.
Insurance does or does not depending on the policy, but whether or not it does require both signatures depends on what the policy is (you can add anyone to your car insurance without their signature. You have to get their signature to add yourself as a beneficiary to their life insurance).
Being married generally does not let you put your spouse on the hook for something without their signature. The only big exception to this is you get to act on your spouse's behalf if they are incapacitated. But even then the government/company/whatever will often insist on waiting for your spouse to recover or die before proceeding.