Better question... why would I?
I've been with the same woman for nearly 15 years. We're not married. Why fix what ain't broke? We've personally got no interest in a wedding, primarily because we believe our relationship is no one else's business. Why do I need approval from the government/religion to deem my relationship more legitimate?
Also, the whole "Till death do us part" bit is played out. A significant amount of marriages end in divorce. People change and grow. Sometimes together, sometimes apart. I don't know what I'm going to be like in 10 years, let alone 20, 30, or 40 years. Why make some arbitrary promise when we really have no idea what the future holds. Me and my partner are together. We love each other. That's it.