As a former F/E I have never understood why the F/E and/or L/M are any less entitled to hold a Commission. In my civvy employment in the role the F/E and F/O were noth on the same pay scale with the F/E often earning more than the F/O by dint of seniority increments.
A reverse side to this is why commission a FO ?
Once you answer that then you see why the 3rd man in the cockpit must hold a lower rank in a rank led environment. The pay scales then are secondary since they follow rank.
The civvies don't necessarily have the same dependence on pay scales following rank.