You can amortize that cost by just reusing what work you already are doing for making the actual device.

Dev kits for game consoles typically started shipping well before the actual device hardware did to give external developers enough time to create launch games. It wasn't uncommon for the dev kits to go out a year or more before the console shipped to retail.

This is particularly true in the era before consoles were basically just locked down x86 PCs and thus developers needed access to a variant of the actual hardware to port their games/engines.

That doesn't contradict with my point. There are also logistic advantages in making the device small.