The actual success rate(people buying something in response to the mail) of Spam Emails is really, really low. Its only profitable for Spammers because it costs pretty much nothing to send millions of emails. Now, if you actually paid someone to write these emails by hand, sure your response rate would be better but I seriously doubt the whole thing would still be profitable.

Also, I think that personalized adds and recommendations(Amazon,Google,Facebook,etc.) have a much higher success rate, simply because they have more information about you and can therefore recommend products that you are more likely to buy. Spammers could of course also collect information about you to personalize their Spam, but considering that most add/recommendation systems work automatically(and are pretty good), I don't think paying someone to create these recommendations would be worth the effort.