Because even if people happen to give you a functional email address, it isn't necessarily their email address.
And I say that as someone who has come to regret registering a first-initial-last-name gmail address. And it's not even a particularly common last name.
It is amazing how common this problem is. I assumed it was incredibly rare, but I have 23 different people who have given my email address to someone thinking it was theirs somehow. Not like "I am just signing up for some forum" kind of stuff, logins to government websites, banks, car dealerships sending me stuff about someone else's financing, etc, etc. It is crazy how many places don't verify the owner of an email address before sending it sensitive info.