[avar]

Because at the end of the day, marriage is primarily about economics.

You can be in love, be in a stable long term relationship, raise children together etc. without being married.

Being married means entering a trilateral pact between two people, and the state. It can make sense to do so for a lot of reasons, e.g. tax benefits (in particular in the US, as opposed to many other OECD countries).

That people conflate this arrangement with the government with what's going on in their personal lives is just fluff. It's understandable given cultural inertia, but if you really get down to it that's all it is.