Because not everything is possible without marriage. Marriage gives a legal framework to so many facets of life that is can be tricky when one doesn't. For example, medical care. If you aren't married, a "spouse" may not be able to make decisions for your care.

And that's just for folks already living in the same country. I couldn't live with my spouse before marriage because we lived in different countries. While one allowed a fiance visa, it was only good for 6 months, during which the fiance not only couldn't work, but couldn't open a local bank account.

People can't control how they feel, so it's unreasonable to expect someone to stick to something they're no longer enjoying. But people can control whether they stick to their word in financial dealings.