You don’t need that many customers to design and ship an SBC. The hobbyist market is enough to sustain these companies. They can sell accessories to increase their margins. Their contract manufacturer can incrementally build batches of the boards.

Many of these boards serve as development boards for the parts they include. If you want a dev board to try this part or you need a cheap RISC-V system to test RISC-V things on, buying one of these is an easy choice.

> You don’t need that many customers to design and ship an SBC.

Right. An SoC takes five years and tens of millions of dollars.

An SBC using that SoC can be designed and made in three months for the cost of a small team's salary -- possibly even just one person e.g. see Paul Stoffregen of Teensy fame, or Jonathan Oxer at Freetronics. I'll bet the teams at Sipeed, Banana Pi, Orange Pi etc are pretty small too.