There's a social trend going on regarding marriage.
Increasingly, the affluent choose to marry, whilst the less affluent don't. Of course there are lots of people who don't fit this trend.
So I wonder if the OP simply doesn't know many married people? She may either be quite young, so her peers are not married yet and she is unclear why older people have got married. Or possibly she's older and just doesn't know many/any people who have married.....so being married and the benefits people feel they get from it, simply aren't within her experience.
I think that as we move into the future, this social trend will become more pronounced. Those who are more affluent will continue to marry - sometimes for the practical,male gal and financial benefits which many have mentioned, some for social reasons - in the sense that it is the done thing in their social and class group to marry, many for love and reasons of wanting to express commitment to the world. At the same time, marriage will become less and less common amongst those who are less affluent - once it is no longer the norm for peoples parents to marry and whole groups of young adults don't really know many if any married people, then they won't do it either. They won't be able to see the benefits. And for the less affluent, there might feel like less financial and legal benefits, or they might not be aware of those benefits. And because they haven't seen how being married often makes many people very happy and how they feel it is different to not being married, they won't choose to marry.
It's happening already and it will continue to happen.
Of course I'm aware that many affluent people also choose not to marry and the choice not to is a trend across society...but it is more pronounced amongst the less affluent.