Why do most marriages fail?

What @SW-User said, plus i'd also add health issues. Sometimes they can bring couples closer together but other times they can cause stress and end the relationship.

People don't learn who they are, marry a person that hasn't learned who they are either. Unhealed wounds mean we seek out others that remind of us of our unresolved wounds. Recipe for disaster.

Lack of communication
Financial problems
Sex issues

Second marriages fail at even ridiculously higher rate than first marriages. On face of it, this fact seems to discount the oft cited "They hadn't learned about themselves yet" or even "All people change" at least as primary reasons.

I think the most evidence supported answer is "because they can"; in fact divorces only became a commonplace, one might say epidemic, after the passing of the no-fault divorce law. I'll hasten to add it's clearly a GOOD thing that people who don't get along don't have to. Of course, the development of the institution of marriage itself is an interesting subject worthy of a book or two in itself; originally a business contract, for example in the European Middle Ages the church publicly allowed men to have a wife for marital duties and romantic partner (So-called "Courtly love") on the side.

An oft cited explanation which I think is somewhat compelling is the "Disneyification" of marriage, where people hold simply unreasonably high expectations. This also leads to disproportionate number of marriages out of desperation or belief in fate. For example, the concept of "soulmate" really only started to be used in 80's.

Lots of reasons. Marrying too young, lack of things in common, different sex drives, etc.

Dishonesty about why you really wanted to be married

different ideas of how a marriage should work

lies

bad communication, or being unwilling to communicate with a spouse who reaches out to you

ignoring your spouse

not fulfilling promises to spouse and family, etc.

People change over time.
The things that brought you together rarely last.
Life is hard. Trying to keep on 'the same page' is difficult. Especially where jobs or careers are concerned.
Add to that money worries; emotional disconnect; the 'failure' of 'dreams' as you go through life together.

Because people take forever to mature into their own.

You can’t hold a steady relationship if you can’t define yourself.

They look around one day and think this is not how it is on tv.. I should be rich and have a socialites lifestyle... ?
Spitballing*

Abuse
Your partner changes from the person that you thought you were marrying

Beacuse they aren't in a relationship with me

A ton of things but in my mind it's they really weren't ready or the person they married really wasnt the one that was right for them or each other in the first place.
If your really with that person you want to be with it shouldn't ever be a problem to communicate and adjust to one another's needs equally.

It's such a hard thing to find ultimately.

Because marriage is no more genuine as it was used to be once.

bad communication, perhaps.

They were punks in a rush

From what I know is infidelity and money (that includes gambling)

D women don't listen

She farts in bed

Fools rush in