Yeah, grandkids. I have five. Too bad you have to go through your own children to get 'em, though. 
But, you get to spoil them. And there are NO RULES. You can even give them the wrong food so they get too fat. Plus, when they get way too spoiled and impossible to deal with, you get to give them back to mom and dad. And the kids love you for it!
Hahahaha,heeheehee. Snicker.
'Scuse me, I gotta go blow my nose. My own nose.
You forgot not having to change poopy diapers and not having to wake up at 3 am to feed the baby.
I wanna be a grandparent!
Robin
No, I didn’t forget. 
I think they do that, because if they don’t people’s first question will always be “how much does baby weigh?” It’s a vicious circle 
Telling people on the birth annoucement probably reduces the number of times they have to answer the question by 5-10%
People may be interested for another reason - if you want to give a gift of clothing for a newborn, you need a reference for size. But mostly, it’s one of the few things one can ask about a baby and get a real answer.
My dad weighed 14 lbs. at birth and was premature…back in 1929!
Back to the subject. People just do it because it’s always been done (weight being so important back in olden times). Can you imagine introducing new employees to the office that way?
“I’d like you all to meet Marion. 5’7” and 215 lbs. Whoo Boy! And this is Bob. 5’3" and 137 lbs. His little hands are perfect!"