That's pretty much the question I just asked.
Ah, so I see.
It's a good question.
I do have a thought as to the answer, but I can't remember my source and didn't really want to try to prove a point when I'm unable to cite any evidence. But I can still bring it up for discussion.
I was reading (somewhere) that in America fewer black people will take up higher learning in the form of college degrees because they perceive that it won't benefit them. The theory goes that if persecution will stop them from getting the job, even if they do get the degree, then why bother wasting a lot of time on doing it. (and unfortunately there is actual evidence showing that persecution does still stop minorities getting certain jobs despite America's reforms and efforts on this point).
I don't really know the situation in Rotterdam, but I expect a similar process might be occurring. Why learn formal dutch if it doesn't actually offer you a benefit? Is your time better spent learning dutch for a job that you'll get turned down for eventually anyway? Or is it better spent getting a job at the local corner-store selling hotdogs to other streetspeak speakers?
(I have a vague memory that this may have been in a book called 'The undercover economist', or possibly a different book called 'Filthy lucre' but I'm not 100% certain that it's even one of those two).