The wedding industry is an expensive mess
came here to say that, not forgetting the social constraint: people may not want to become a 'married couple'.
I answered in the post the OP mentioned why my spouse and I made our 25+ years old (and counting) relationship official and it's out of pure legal considerations: giving the other full authority in case of medical emergency, protect the other I when the other will pass away (and spare them the burden of having to pay taxes on the inheritance), making it simpler (and safer) to rend our buy a home,... BUt it still took us years to decide doing it: because we're also one of those (odd?) couple that have chosen to live our life like we wanted to live it, not like society (and friends and families) expected us to.
Edit, if I may:
Women tend to live longer and happier if they are single.
Do you have any data backing that up or is personal feeling? I would be curious to read more about that.