Why do they do this?

Marriage is a Contract with Legal significance where as living together has no such legal binding . Those who live together unmarried has no regard and respect towards Law. They believe contract of marriage is only a piece of paper . Main reason is ignorance of law . The impact will be on the children . The society treat them as bastards . 

living together as a couple without the "legal document" of marriage is called "infidelity".    and only immoral people who do that kind of lifestyle....

Because women changed divorce laws to take everything from men and destroy them, why would a man marry unless he is stupid?  The mans goal is not to marry and to convince the woman its the same as being married. The trick is finding hot stupid women.

Part of the disintegration of society the whole world is experiencing right now is the bastardization of language. So there may actually be people who don't understand the true definition of these words. We've gotten incredibly daft as a species. But amongst the people who do know what these words mean, sometimes it's wishful thinking, as in one wants to be married but their partner refuses.

I did that for years called my living girlfriend my wife  and feel if I had done with my ex it would have helped make things better 

after 20 years we did get married ,,, no wedding so no wedding photos 

you are  way over thinking things 

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comes under the point when you call them OUR UNDERWEAR not his and mine

if you share them I guess they are ours 

Her husband is her partner, or her wife, who cares...And no one is interested in 50 year old wedding pictures, unless, you are looking for a reason why your life is a complete disaster and you have squandered 50 good years. 

When people start using those terms, to me it signifies that marriage means a great deal to them... but their partner doesn't feel the same way. Yes, those are all legal terms. Not "endearment" terms. 

I agree and, from what I can tell, so do most people.  Words do have meaning!  For example, when someone calls their boyfriend's parents their "in laws", odds are very high the parents don't like this.  The wife of their son is a family member and this is a big deal to them.  Their son's girlfriend is just that...a gf.  I've also noticed people are more likely to do this when kids are involved, which is really unfair to those kids.  

Also, people saying marriage is "just" a legal thing are completely wrong.  It represents the biggest commitment 2 adults can make to each other, and this goes well beyond legality.

I think it’s the sake of convenience. People hear the word “husband” and know what it means, but the word “partner” (which is often used outside the U.S. to indicate a long term unmarried romantic partner) inside the U.S. still carries an impression of a same sex partner or perhaps a business partner. Same with the term “in-laws”, when it is used people know what it means and it’s more convenient than saying “my boyfriend’s mother” or similar.

Personally, I agree with you. If marriage is only a bit of paper, then go get the bit of paper. But that bit of paper gives me certain rights and privileges that are not automatically given to an unmarried partner, no matter what you call yourself. If you don’t want the bit of paper, that’s certainly your right, but don’t present yourself as married. And as of 2020, only 8 states still recognize common law marriage.

you know that marriage is just a legal document.

for thousands of years, marriage was recognised as a commitment made in front of a group of your peers, with a blessing from the god of your choice.

In modern times, it was all about proving legality, which is why we now rely on a piece of paper to declare a marriage - and there are some good things about that.... but it's not the be-all-and-end-all of marriage and why shouldnt we consider someone who has made a loving commitment to someone else, just as 'married' as a couple who signed a bit of paper?