Perhaps I’ve missed some obvious reason why the industry still does this.
Because if you get a random email from some site you've never signed up for, there are two possible scenarios that you cannot distinguish between:
1) Somebody has maliciously signed you up to a legitimate site. 2) A malicious site is trying to get you to click a random link.
This proposal suffers from a common flaw, in which people assume they can change just one thing and have everything else in the world stay the same. Systems don't work like that.