Those boards have a lot more on the board than just the cpu. At a minimum they have power conditioning and ram, usually also storage. A lot of what you pay for with an sbc is that routing and layout. If it’s got WiFi as well, you could be paying for the testing that goes into rf micro strips and potentially certifications on em emissions.

It is, of course possible to do all that yourself, but the system on module exists, because this integration has value that people are willing to pay for.