I wish those SOM vendors can produce smaller and cheaper carrier boards with their popular SOM installed, basically becomes a SBC with SOM, so I can start with them before buying more SOMs down the road.

RPi did the right way in my opinion: its (cheap) SBC format gains attention widely, then it started to sell its own SOM in large volumes.

We (as an industry) do produce some smaller SoMs. Long are gone the days of gigantic EVKs with a thousand interfaces, but it's costly compared to RPi, because of scale and all the certifications we make our hardware go through.