a RPI is just a SBC with GPIO pinning (which isn't at all unique to RPI.).

The main reason it was/is the popular behemoth is not because of it's tinkerability, but because it got mainstream appeal and RaspianOS is polished enough for the average windows user to go "wow, neat."

THere is no "knock off pi", there are many many popular SBCs with the same performance at cheaper cost (O-Pi, arduino), not to mention the mini PC space (NUCS, ie intel asus.)