Actually e-mail is guaranteed to either be delivered or bounced. It's more reliable than the USPS.
Yes, but then you have spam filters, automatic diverters in some clients that shunt the e-mail to 'holding' folders, and the ultimate selector. A recipient that simply refuses to click on the e-mail, or who just erases it.
When you receive dozens of e-mails at a time its easy to justify not touching them, simply because you sometimes just don't have the time. Not the best thing to do if you're running a business, but it happens.