Dev board prices are basically random. Disregarding these parts in particular, microcontrollers are a good option because they keep the process of actually bringing something to market simple and cheap, especially for a hobbyist. To go beyond an STM32 or similar (Kinetis, ATSAM, etc.) to an MPU, you end up having to use BGA parts which means board prototyping/manufacture is harder, you have lots of extra PCB design work and you'll probably be putting yocto or some other linux flavor on it which sounds nice but can be a pain. A microcontroller is one part, usually with exposed pins that are easy to hand solder, that you can stick on a PCB with a voltage regulator and maybe a crystal and have a fully functional design. You don't need to worry about board feature sizes/tolerances, high frequency/controlled impedance traces, and the mess of support circuitry that a basic MPU will require.
I know people like to write them off because it's so easy to get devboards with 1GHz+ MPUs and gigabytes of memory/flash on them but if you want to sell stuff, eventually you need to package the design and if you don't NEED linux, you can make your life so much easier and in most cases, have a much smaller product with way lower power consumption.