RPi as a board is clearly simple enough that there would be clones if you could get components at competitive prices. The only conclusion is than that you either can't get the CPU, or you can't get it at a competitive price.
RPi use Broadcom chips, which you rarely find these on other hobbyist boards. The most likely cause, after what I heard from people in the industry, is that you usually either can't can't get Broadcom chips at all, at acceptable prices, or even documentation for them when you are only interested in the small quantities you'd need for introducing a new/clone hobbyist board. Not unless you've got some serious connections.
The only reason I would consider buying an Arduino (or similar) is for super-low-power applications.
Otherwise RPi would have been copied a long time ago at a lower price.