But Flash has a limited number of write cycles. It's a big limit - 100,000 to a million writes is typical. But it can happen. Magnetic platters have a designed failure rate of 10^-13 to 10^-15.
Too put that in more useful terms, you could write to the hard drive ten times per second for 31,700 years before your odds of getting an error became 50/50. Of course, hard drives also tend to be ruined by impacts, which happen when people drop their devices. I like Flash better than hard disks, but the engineer in me acknowledges that in certain situations, hard disks are a better solution to certain kinds of problems than flash memory.