Pis/other SBCs and ESPs/STMs/etc are effectively a different subclass of embedded
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
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