Why do people say "I love you" then get married?

My (19M) gf (18F) says that she loves me. She has plans on making our relationship long-term and we've had very few issues. She's also very close to my family. I know that her mother wants her to have kids, but she said she doesn't have to have kids if she doesn't want to have kids. And she's also the only girl I've ever been in a relationship with. Honestly, I don't see the point of marriage unless you're having kids. This does work both ways btw.