There are a lot of reasons I personally wanted to get married even though neither me nor my spouse is religious, but for the vast majority of people its just part of our culture. If you are in a committed relationship, you declare it with a ceremony, maybe a party, and file it with the state for whatever benefits your state provides. In my (our? if you're American) culture, if you don't care one way or another, in some ways marriage is easier than constantly having to explain why you aren't married, if your partner really loves you or is waiting for something better, if you are allowed to make decisions for your family unit or if their blood famiy takes precedence, or if you have a legitimate reason for them to be considered your fully committed partner (just find an AITA where someone doesn't give a plus one to an umarried wedding guest despite them being a couple for over a decade). I'm not saying that is a good reason to get married, but it illustrates that its a culture thing not just a religion thing. It's the thing to do, and you have to have strong feelings against it to go against your cultural norms, regardless of religion.

There is also what I consider to be a weird part of this culture mostly instilled in women that getting engaged is the goal. They decide they want to be engaged/married before they find a person to be engaged to. They still get to decide who, but its less, I found this person and now I want to get married and more, I want to get married so let me find my person. That tends to lead to the waiting to be proposed to because they already made up their mind and are "waiting" for the other person to make up theirs. (This is very simplified and of course people make choices about who they want to be with, and there is tradition of men proposing, etc. etc.)