Why don't switches appear in circuit diagrams of DC motors?
The main reason that your "switches" are never shown in a principle schematic is of practical nature. There is no need to show all the details for a good understanding of the schematic. If the subject changes to understanding the actual construction of a DC motor or generator and the types of winding involved more details are given.

The inductor in combination with the rotor in your diagram represents a socalled serial DC motor. Here only the principle of the motor is shown and not the actual coils in the armature or stator (L). Depending on the design there could be many coils and brushes. In an electronic equivalent the brushes are replaced with semiconductors. 

When an armature turns in a magnetic field (L in your figure) a back voltage is generated. That voltage is in average constant at constant angular velocity of the armature. For each individual winding this is not the situation. In practice the commutator "switches on and off" and changes the action to a different winding. In fact the voltage induced in the winding has a sinus form.

Since there is a fixed field in the drawing this type of motor is a socalled shunt motor. The field has thereby a constant voltage applied although it is possible to make the exitation of the field adjustable to regulate the velocity of the rotor. Less exitation results in a higher speed.