The main reasons are the lawnmower manufacturers don't make the engine or transmission. So that just leave repair info on how to change belts, and diagnose electrical components, steering components and the frame and associated parts. Then it is assumed that the standard technician is proficient in minor electrical testing, and how to turn a wrench.
Basically companies like Husqvarna assume since their repair dealers have had engine and transmission repairs that should cover the information needed to be able to repair what is left. Last track I had MTD required that all new dealers be certified in Briggs, Kohler, Honda engines, and hydro gear transmissions, and have engine service association certification in 2 cycle, 4 cycle, and hydraulic components.