There's a simple reason why stranded wire in smaller cross sections is not used here. Sockets and switches with screw terminals are flat out extinct and push-in terminals are not rated for the use of stranded wire, neither with nor without ferrules. And even if there were screw terminals, crimping ferrules on every single connection would take so much time it would be economic suicide.
Last but not least, stranded wire is thicker than solid of the same cross-section, I can only assume this is due to the (albeit small) air space between the strands. So, the notoriuosly small Austrian/German boxes overfill even quicker (I once tried to correct an install where a moron had pulled 3x2.5mm2 stranded PVC cord in conduit). Solid wire needs a strong push, but then it's in the box and will stay there.
