Why do some people live together and never get married?

Kimberly
I am genuinely curious about this. I see some people who have been with their significant other for 15 years with multiple children. I've seen some together for less or more with no kids or 1+ kids. When you're together and living together for that long, how come you don't get married? 
I don't really know how to communicate my sincere tone through text but please know I'm not judging or acting like I know a better way or anything. I really just want to know what all may be factored into a decision not to marry at that point in a relationship. 
Thank you for the information! :)
Edit/PS: thank you all for your responses! I appreciate the kindness and respect you showed in the responses whether you agreed or disagreed with marriage. I am grateful to those of you who shared your personal experiences as well. Thank you!
After reading everyone's posts, it seems the top reasons people don't get married are:
-not religious/just a piece of paper
-too many marriages end in divorce and don't want the same
-have been divorced, bad marriage experience 
-finances 

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