There are smart parking garage lights, smart inspection cameras, smart golf ball dispensers... all these things have enough margin they can absorb a 100-300% component increase, though I'm sure they don't like it.
Pi - 73 million sold
ESP32 - 1 billion sold
STM32 - 13 billion sold
For every 1 Pi sold, there’s almost 200 STM32s sold.
Firstly, Pis and SBCs like it tend to be fully functional OOBE on their own. This has useful properties but also means that hooking them up and testing their use is a little bit simpler even compared to a microcontroller dev board
Secondly, it's a full Linux ecosystem and all of that that it entails. This ecosystem has more in it than these ecosystems (especially in the FOSS world) and it's also useful for projects that exceed a few MB of RAM. Sure if I want to do a few things a microcontroller like this is a very very good and probably the better option (it isn't that difficult to write a lil C to control these things) but SBCs can do lot more than that while still keeping many of the advantages, which may be the difference in some cases