Why cob and why not concrete?

Before I build my RMH, Im wondering why the use of COB rather than concrete/mortar/ect since its easier to obtain/make concrete.

Around 400 C (below dark red heat) the cement in concrete starts decomposing. Also cob is better suited to withstand expansion and shrinking. That's why fire bricks are made of aluminosilicates (or other substances) and not cement + aggregate or lime + aggregate.

Concrete is ok for in the lower heat parts and makes a very good mass but it must be vibrated concrete without air bubbles  if you want to get the best out of it.

Even when used in lower heat areas where decomposition is not an issue, concrete is forever and you will not easily be able to modify it in the future if you find that some part would be better if different.