I promise I don't mean this in a snarky way...

interesting question:

it could be because a marriage implies a growing family. Congrats your getting a new son or daughter in law. or even eventually a new grandkid

it could be congratulations - you did your job and raised a kid someone else likes. and congrats (presumably) your son or daughter picked an upstanding member of society to be in your fam.

it could also date back to a tradition where the brides parents gave the grooms parents 🐑 sheep in éxchange for marrying her. congrats - you got a new sheep.

i think it’s generally just a happy occasion for parents and kids so they just congratulate them that such a happy thing has occurred to them. it’s exciting for most people when their kids get married.

Its very much, 'Congratulations, at least one of your children grew up sufficiently normally to be accepted by someone else'

I admit
Yesterday I wanted to say to a dear woman announcing her son's upcoming wedding
"now your troubles will multiply"

in stead I said "congratulations"

she was so happy

.......................

maybe passing on a difficult boy onto another woman is like winning the lottery

It's very common here to congratulate the parents 🙂

I don’t know
But that’s a good question

It’s a knee jerk response I think.