Don't they realize that people would possibly, oh ... I don't know ..., CALL THE BANK?
They don't care if the smart ones call the bank.
Lets say you have a 1% success rate on getting people to download the attachment, and only 1% of those downloads are not stopped by the computer's antivirus software. That means you're infecting .01% (one in ten thousand) recipients. Now, let's say you email that scam to a million email addresses (sent from a list harvested from online surveys, mailing lists, etc). If each of those hundred computers you infect can get you an average $100 profit, you've made $10,000 with very little effort.
Oh, and you now have even more email addresses because you've harvested the address books of your victims, so you can sell those addresses (as well as adding them to your list of potential victims.
Just think if you could get someone to download it on a company-owned computer or a public computer (say, in a library) because that's even more data that can be harvested for potential profit.