They are manufactured with individual memory chips, so they can use 8 chips to get one capacity, or 9 chips, or whatever. Plus the devices contain more memory that the user is allowed to access. The firmware of the device resides somewhere, as well as the extra space that the device uses for its own housekeeping and so forth.
RAM chips have probably stayed with the "power of 2" thing more because of how we looked at memory and what the users expected to see more than because of any limitation in how they are designed and manufactured. Hard drives have always been a little more loose with how they specified capacities. Look at all of the different SSD sizes available on newegg- there are even more odd numbers such as 90GB and 115GB and so forth. They go from 30GB up to 256GB in barely 10-20GB increments.
It is a matter of advertising. When you see an ssd advertised as a 64GB ssd it refers to the total raw capacity. The total raw capacity is not the same as the available capacity. The actual available capacity will be less for a variety of technical reasons. This led to complaints from users.
In response, some manufacturers and vendors started rounding down and advertising their ssd's as 60GB. That may or may not be the actual available capacity. It can still vary for a variety of reasons. The variable that is mentioned most often is over provisioning. The percentage of space that is reserved for over provisioning can vary quite a bit.
For all practical purposes a 60GB and a 64GB ssd have the same total raw capacity while the actual available space will vary.