Well, your ARM-powered chip will need some analog-to-digital converters since the feedback signals are analog. And you'll need a high-current output driver to c

Well, your ARM-powered chip will need some analog-to-digital converters since the feedback signals are analog. And you'll need a high-current output driver to control the switching transistor. And some sort of startup circuit to provide power before the processor starts running. And an undervoltage lockout circuit to prevent the processor from malfunctioning if the voltage drops. So you still have a bunch of analog circuitry but you've added an analog-to-digital converter and a microcontroller. There are some applications where this tradeoff makes sense, but in a lot of cases it would just be excess complexity.