Aight, you seem to want to ignore the legal benefits, so I won't mention that beyond saying that it is a hell of a lot easier to get married than to figure out all the paperwork needed to duplicate it, and not even have the exact same outcomes, just the majority. The tax thing, for example, you can't file jointly if you aren't married, no matter what else you set up (edit: in places where things like common law marriage aren't recognized)
The biggest thing is the experience, imo. The memory.
Now, me and my wife went to the JoP, with our kid and required witnesses (my best friend and his husband).
No fancy reception, no major party, just went home and said to my dad "we're back, no problems." He said congratulations, and went back to watching TV.
Total spent was about a hundred bucks, including gas. And the memories of it are wonderful, we cherish it all, and we're happy as hell we didn't do anything else.
Wedding ceremonies, however, are expensive once you go beyond that bare minimum. That's a cultural/sociological thing where the needs of the individual and the culture mesh into not only believing it necessary, but beneficial.
And, for the people that want it, it is beneficial. Ceremonies, rites, rituals, they serve a purpose beyond the legal or official status that comes with them. Weddings are as much about community as they are the couple. It's the union being both recognized and celebrated at the same time, even when it's a secular ceremony rather than religious.
Don't get me wrong, the money spent on empty bullshit surrounding weddings is absurd. But the actual wedding, where the community stands around the couple is incredibly powerful in terms of validation, even when it's the license that really matters legally. You can have ceremonies without the license; I performed several of them back before same sex marriage became legal. Those events were important, and doubly so because they had no legal standing.
I think that's what you're missing, that there's a massive difference between two people shacking up and marriage. When the people involved swear an oath, and/or exchange symbols of union it means something, even if there's no witnesses, not even someone to perform a ceremony. But as you move into witnesses and an officiant, it feels different because it is a public commitment. You can still divorce or whatever, but it happened, and you can never deny that. That moment, the vows, they exist in a way they don't if you swear only to each other.
Yeah, two people can be just as committed, and honor their commitment perfectly without anything else. But it feels different.
Now, again, I'd argue that once you start shelling out for crazy dresses and cake and niche receptions, you hit diminishing returns very quick. That's to satisfy other things, not the union itself. It may well make people happy, but it doesn't add anything to the underlying point of there being a ceremony in the first place. That of saying to the world "where once there were two, now there are one".
Not that anyone has to share the valuation, but it's what underlies the whole thing, and it has value