P2PPCB platform is mainly for prototyping, partly for bespoke / handmade keyboards, and never for mass production. So the production quantity of P2PPCB components will be quite small. It makes the cost math very different from mass production.
The symbol of the small quantity cost math is Raspberri Pi Pico on mainboard Bob / Charlotte. It may look odd from the viewpoint of mass production, but it is reasonable for P2PPCB.
The unit price of Raspberry Pi Pico is 4.00 USD (Digikey). It has micro USB type B connector, tactile switch, 2MB external flash memory, power regulator, LED, and RP2040 (MCU). The BOM cost exceeds 4.00 USD for small quantity production.
Another merit is 2-layer PCB. Mainboards of full size keyboards require a lot of matrix wires. 12 wires for column and row each at least. More wires can be required for many reasons. So mainboard Bob / Charlotte have 12 and 16 wires for normal column and row. And such many lines often require expensive 4-layer PCB. To accommodate such many wires in smallest 2-layer PCB, we must have careful planning and clever hacks. Raspberry Pi Pico helps the job much because it shoulders USB and power regulator circuits. Thank you Raspberry Pi Foundation!
The figure below shows a hack.
74HC164 is an 8-bit parallel-out serial shift register. The order S6-S4-S2-S0-S1-S3-S5-S7 looks odd. This is for minimizing PCB. The figure below tells it.
As you can see, through hole connector header is not very good to minimize PCB. However surface mount connector header easily comes off when you pull a connector.
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