Because, for instance you have to be over 18 to get a job, because your minimum wage could be indexed to age, because your CV will give most of that away anyway and because various official forms your employer will need to fill in accurately contain a field for 'date of birth'.
You probably can't even open a bank account without specifying your DOB, let alone get a job or a loan.
I know people stick their DOB (and photo!) on their CVs in the Netherlands. As far as I'm aware that would be quite odd to see in the US. There are some differences here in what is comfortably or preemptively shared with employers between the US and the NL.
It's still the usual way of things in Germany. You bet I had a nice professional photo taken. You list not only your birth date, but birthplace and marital/family status.
Job-hunting as a married-but-childless 30 year old woman here is awesome. Not. When I was stupid enough to make the whole "why do we list our family status on our resumes" thing a point of smalltalk towards the end of one of my first interviews here, the guy cheerfully told me, "oh, because we need to pay a family father of 3 more than we need to pay a single guy"
... and the young, childless wife of an engineer working at that small city's most generously-paying employer? The guy did not seem to believe I needed a job at all. All's well that ends well, and I'm now in a far better job that has a relatively transparent union payscale, but it stung at the time.
My husband, a German, has told me that you used to also list the occupations of your parents!
Good grief, that's incredibly off-putting. I've refused to add a photo and DOB to my CV on principle and fortunately it hasn't seemed to hinder my job seeking.
