Linux

Mini PCs / old Optiplexes don't have built-in GPIO and a huge database of kid-friendly projects to do with them.

It's that software and community and hacker ethos built into the design that makes the difference, not the performance specs of the hardware. The point is "facilitating kids learning about computing," not merely "being a desktop computer."

I mean, I guess you could install Raspberry Pi OS on a PC and add a GPIO breakout board (like this or this) to try to replicate the pedagogical experience of a Raspberry Pi without the "overpriced" hardware, but it's not going to be nearly as straightforward as just using a real Pi.