Why are current sources not drawn explicitly in the datasheets' schematic diagrams?
Most likely just to simplify the schematic.  They would likely be implemented with various scaled and interconnected current mirrors as well as some sort of reference current generation circuit.  One complicating factor is that this is 1 channel out of 2 on the chip, and it's entirely possible that the biasing circuitry is shared between the two channels in various ways.  

It's common for the biasing circuitry to be relatively complex as it has to provide lots of different currents to different parts of the circuit, and maintain accuracy despite voltage and temperature variations.  However, the biasing circuitry simply ends up generating some well-controlled reference voltages and currents, so drawing those on the schematic as sources makes it much easier to read and understand the important functional parts of the circuit.