Because a low voltage power supply for the micro is quite expensive generally, compared to everything else. Plus you're probably adding a PCB and more BOM parts

Because a low voltage power supply for the micro is quite expensive generally, compared to everything else. Plus you're probably adding a PCB and more BOM parts. There are some ways to make cheap non isolated supplies, but then the microcontroller can't be wired to any external low voltage I/o. Of course, if you already have a PCB and power supply, then it makes total sense and most products that meet this criteria are microcontroller based.