Because marriage has nothing to do with love. It's just a social construction designed for the purpose of disconnecting us from our true selves and creating friction between men and women. You never know how long you're going to be attracted to/compatible with a person. It can last for a few wks, months, years. You just don't know.
I've never been married but did live with a man for
eight years. Nothing bad happened, we just grew apart.
Since I've been living alone I have peace I'd never
known before and am more in touch with my true spirit. When you live with others - lover/family/roommates, it distracts you from self-contemplation.
Most people just don't want to be alone and ignorant societal pressure is still rampant. Still, people still think there's 'something wrong with you' if you're not married. Or they think you're gay which is the most ridiculous ignorant thought because I could just as easily say I don't want to be married because I want to have sex with a lot of guys. Not saying there aren't people who haven't been happily married for years, but most married people I know are not happy. Even if you do find someone you're happy with for the rest of your life, I don't see the purpose of 'legalizing' it and 'setting it in stone' which is all just based on insecurity and societal pressure.