The main cost of getting married is getting divorced. Legal fees add up a lot here, and the division of assets is costly.
Now, you could argue your marriage won't end in divorce, but 60% of marriages in the US do.
60% of marriages end in divorce, but most people who marry do not get divorced from their first marriage. The 60% number is bumped out due to the segment of the population that get married 3 or 4 times.
Anecdotally, I know a lot of people who are either divorced in their 30s, or still unmarried in their 30s. Considering the difficulties a woman has conceiving beyond their mid 30s, I think the explanation is here in our faces.
My grandma had 5 kids and was married at 19.
Funny that you've mentioned correlation and no causation, and then brought this up. How old was your grandma when she had her first kid?
We were going to use the church hall for the reception, but my parents wanted us to get another reception venue. That would have been free. A traditional cake and punch reception with some music would have set us back maybe $300, but even that could have been foregone. My wife's parents didn't do any of that. Just got married the old-fashioned way.