"It's typically an expensive ceremony"
A 'marriage' is not a 'ceremony' or a 'piece of paper'.
It's not 'typical' for a ceremony be 'expensive' - that's a very modern construct. My grandparents went to the Church, got married, there was a banquet at the hall, people brought food. That's it.
But how hard is it to understand that people want to honour their commitment in some way?
"Why do we have big awards ceremonies for those people (artists, citizens, soldiers), it's so expensive!"
Some (most?) people would want to honour that commitment. Many people who don't, just don't find themselves in the right conditions, but would otherwise.
But beyond the ceremony, marriage is a social institution, not a 'piece of paper'.
(Personally, I have a hard time grasping why people struggle with this)
A marriage is something 'that is built', the ceremony is just part of that, it takes time, effort, input to build a relationship, it's not just 'something that happens'.