Sole sourcing of ASICs is a necessity of life unfortunately. They’re rarely pin-for-pin compatible, and hardware doesn’t allow quick spins due to regulatory recertifications any time you touch the PCB.
If your widget needs a computer in a small form factor, you can’t do much better than a pi. Not many firms have expertise of high-speed-design, so off the shelf hardware to handle that portion is pretty much a given. The fact it has exposed and broken out GPIO crowns it really as the only option in the world of SFF PCs. Arduino-class products are microcontrollers, the ATmega328P that powers them is an extremely basic MCU borrowed from industry.
For reference the least expensive industrial pc is $160 on digikey. The other SBCs are from non-reliable brands without certs, volume, etc. Rockchip based designs are going to be a no-go for many due to security implications, however imagined they may be.
These industrial customers may have started off a decade ago as a hobbiest on the pi; their volumes ultimately keep prices down for the hobbiest.
Which fast and furious plot line was this again?