If marriage offers protections, why is a pre-nup necessary?

I’ve seen posts in this sub and other similar financial subs where people say a positive of being married is it offers both people protections over their assets that may not exist pre-marriage. I even posted a while ago about the best way to split the purchase of a home with my partner, and overwhelmingly people say it’s much riskier to buy a house with a non-married partner than with a married partner.

If there are protections, then why are pre-nup agreements recommended? It seems to me that if you’re worried about pre-nups, then an option (if both people are fine with it) is to not get married and simply be cohabiting partners. But I’ve read and been told that purchasing assets with someone that you’re not married to is risky.

So I’m trying to better understand what protections marriage offer and what they can’t protect that a pre-nup could.

Thanks