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.
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.