Sadly, like too much engineering writing, no attention is given to the "why".
I've just given up giving a full explanation and now just point to this:
http://arstechnica.com/tech-policy/2013/04/how-a-banner-ad-f...
And this:
https://www.eff.org/deeplinks/2014/11/verizon-x-uidh
If they still aren't convinced then they weren't convincible to begin with. There's other arguments too, but if the above doesn't sway them then none of the other arguments would either.