Why is marriage a joke to some people?

Some people don't take anything seriously....relationships, work, education, etc.

We (Americans) live in a fast food, instant gratification society where people are used to getting things they want, almost immediately. Where if something is broken it's often easier to discard it and get a new one, than to have the broken one repaired. Alas, relationships are often treated that way too. If it's not working, end it and find a new one, rather than put in the work to fix the one you're already in.

Marriage is only a joke to people who are a joke.

You can't expect everyone to have the same opinion as you. But in my opinion, marriage is a good thing. I wouldn't stay with someone who thinks its a joke.

Marriage should be long lasting relationship but the frequency of breakups hint marriages are used to as friend for benefits.

I don't think it is, at the time they decide to do it. Of course, some people are immature or impulsive, lonely, or just not very thoughtful.

Because they don't understand it, and/or have had others set a poor example of it in their pasts (like by the parents).

Because to them, marriage is just an excuse to have sex all the time. They are immature and they had bad parents.

I see it as a serious thing.