[puzzlingcaptcha]

I'm not sure why the 'pro micro' format is so popular. If you are already designing a PCB, why not plop an Atmega32u2 on it directly (assuming you dont need wireless)? You only need a crystal, two resistors, four caps and a usb connector.

[jsheard]

It's just easier for people with less soldering experience, all the tricky SMD parts are already installed for you. Especially now that QMK has shifted from AVR to more powerful and readily available ARM chips like the RP2040, which are often only available in packages that are difficult to hand-solder. Plus designing a keyboard around the Pro Micro means you can use the same PCB for wired and wireless builds.