Time is short. Actually, that isn't quite how time works. I should say instead we can tend to be short on time. And in that time I don't wish to have to sign up, in, and for everything I am ever to be presented with on the Internet (capital I, because it's muh-I internet).
If a website won't let me read content without signing up, even if sign-up is free, I don't want to read it.
If an e-commerce shop requires me to create an account in order to checkout and make a purchase, I won't be making a purchase from there.
You get the idea.
It's easy enough to get all of my information. I shouldn't have to take the time to enter it so you can send me emails I never signed up for and I can endlessly unsubscribe from them and then you can ask a question as to why I'm unsubscribing that you'd be able to answer without my help if you filled out a corresponding checkbox when adding me to the email list in question and the checkbox would say something to the effect of, "this poor sod just wanted to buy a pair of novelty socks for a friend and now he will be forever bombarded with emails suggesting purchases."
Once you unsubscribe, I'm fairly certain they then keep a reject bin list that they let sit idle for perhaps a few months before re-subscribing you. I might be annoyed to get that email after unsubscribing, but they're banking on enticing me with a catchy headline, or maybe tricking me into opening it, merely so that I can unsubscribe. And mostly they just don't care because they can set it and forget it and I'll be on and off that list without any effort on their part.
Then of course there is the email confirming your unsubscribe. It's all a true delight. And if enough time's not wasted deleting, unsubscribing, and/or setting as spam, well, you could always blog about it. Something tells me I've even done it before but I refuse to waste even more time with a quick search. I can only imagine how you might feel reading this far.
On an uplifting note, here's a tale of a high school girl persevering through the puzzling illness that is MS.