As You probably saw on many SBCs RAM chips are soldered near SOC, sometimes even on top of it. This design makes few technical issues easier to resolve and results in overall lower price point and smaller design. Removable RAM adds some complexity and its for sure much more expensive. Also there is simply not much space on board to get that.
On top of that you have to keep in mind the difference between LPDDR and regular DDR. Most of the SBC SoC’s memory controllers only support LPDDR. There are CAMM modules that solve that issue and there was also an image shared of a prototype once by Radxa but the cost does not make any sense in the end.
And even if those issues would be solved, there is still the issue of training that could eliminate many DRAM sticks from the compatibility list. Even in the x86 world, it’s super common to find RAM sticks that do not work on certain boards but do on others. Early Atoms were notoriously known for being extremely picky about which models would work, and the rare few ARM boards that support SO-DIMM have the same issue.
While I think we (users) would all love such a solution, it would significantly increase development and support costs, and turn small inexpensive devices into much more expensive products in the end, i.e. the opposite of the original goal.
The risk is also that by the time this is all specified, designed, implemented, produced and put into shops and in the hands of customers, the RAM market might have changed again, so it might be a much more expensive total solution and also likely not fast/performant and more power hungry. No-one will buy then.
Also is it still a SingleBoard Computer then ? With several Arm devices needing SD-card or SSD/NVME and also HAT and USB adaptos for storage e.t.c. ‘single board’ is anyway not really the case anymore, maybe only that it is not called a ‘motherboard’.
