But a power supply is an electronic device that can be modeled by Thevenin's Theorem. It contains a diode bridge and active regulating components.
Although composed of non-linear components, the power supply's overall functionality is linear.
I could also ask "why learn Ohm's Law for electronics?" - it's essentially the same question.
Many transistor and diode circuits are designed with Q points. Finding the Q point requires knowledge of Ohm's Law and related theories.
Transistors use Norton's Theorem to model their behaviour.
Even devices which are not linear can sometimes be broken down into functionalities which are linear. For example a transistor switch is not a linear device. But, the transistor can essentially be broken into two states, on and off, whereby the transistor can be replaced by a resistor. A low resistance in the on state and a high resistance in the off state. Effectively, the circuit can now be analyzed with linear equations!