pre-nups are a thing but come with strings, and are not unilaterally enforceable without certain considerations. both sides usually have to have their own lawyer -- often not cheap -- and there are often ways to break a pre-nup.

state laws have an impact as well. a California pre-nup ain't the same as Kansas.

they're also not really useful for many people; yeah if you've got a $4MM startup and a few assets it's good to CYA, but the average household of 2 renters and middling salary might not benefit, while both sides pay out the nose for lawyers.