The relational algebra is a line encoding. When you put bits on a wire (or a magnetic surface), they're not the same bits; there are more of them, for at least two different reasons, and they have to be translated in both directions. But you only want to see your bits. Similarly, programming languages only want to see their data structures, but The Database can only deal with the relational algebra, which is like a wire not being able to transmit more than three consecutive zero bits, and the translations are arbitrarily complex. The difference is that wires and disks don't have administrators who demand that everyone must think exclusively in terms of the line encoding.