I think they say that out of habit. I mean, when I was little, they used to buy me dolls even if I liked construction games like legos, so I learned how to take care of my looks and take care of babies.

when I grew up, they started asking me what boys did I like. After that, they asked me if I had a boyfriend. Then when I would marry and so on. For that people, a woman could only be successful if she married and had kids. That was it.

So if you believe that, then the "you'll die alone/become a spinster" is terrible, since it means being a failure. Since for them all our value comes from getting married and having children, the threat makes "sense". Of course, their premise is false, but they are unable to see how wrong they are.

In fact they refuse to even entertain the idea that marriage and children might no be the most important thing in a woman's life, because that would mean they have been unfair their whole lives.