If some shady Asian company can build these for under $100, then why don't we have a decent open-source phone yet?
Turns out, making the software to do cool stuff is most of the cost of building new products.
With your average Intel desktop almost everything is in the Linux kernel, you don't have cellular, and you use UEFI or BIOS to boot.
With your average Android phone, you are lucky if you have kernel code. This does not include drivers for most things, and the code is of such quality that even when manufacturers try to update the kernel version they are having too much trouble.