BRAH BRAH:
That's part of the problem with Facebook along with the phone number, in both cases it's a violation of privacy and as you said a tool for data collection.
Privacy laws have not kept up with technology. My data should not be used by companies like Linkdin or Facebook, which I have no relationship with. It should be illegal to ask for your contact data on your phone, and not say what it's going to be used for beyond 'looking for friends'.
I still get Linkedin messages sent to my government email address, but my Linkedin profile is disabled and does not include my government email address.

It's because the Government of Alberta publishes it's phone list publicly, and companies scrape that data as 'just another source', because they can.