Because it's not exclusive to religions?
Also, please pardon the language here as I rarely use profanity when I post - but let's drop the bullshit. A LOT of people who are getting married aren't even close to living the way the religion that is the majority in America dictates. That is a very weak argument that has no legs to stand on.
Reasons to get married? Health benefits. Joint property ownership. Some have children from previous relationships and it allows for both parties to have some legal standing in relation to the children.
Marriage also hasn't been only a religious thing well - in my entire lifetime. My mother wasn't married in the church, it was a civil ceremony and it was fully legally binding. When I was married, it was done the same way. Are you saying that I don't have the right to get married if it's not in a church, nor did my mother?