It's a matter of the resonant frequency of dynamic drivers and manufacturer tuning by controlling cup size, spacing and volume. As far as I know there's nothing that prevents closed back dynamics from having a good linear response as long as the seal is good, which unfortunately means that open back dynamics will have an inherent weakness in the low bass.
With planars, and unlike dynamic drivers, the interaction of the magnetic field with the wire set into the diapraghm can drive the entire surface at once if necessary, so bass response is not sacrificed in open back designs.
Edit: The ER4S you mentioned is IIRC a hybrid balanced armature and dynamic driver IEM. The lever-based action of BAs means they will tend to have peaky resonant frequencies and limited range, so are used with multidriver designs with crossovers. Because IEMs are so small its a matter of tuning, gauging insertion depth and driver size, the latter determining whether they will fit in your ear as close to the eardrum as possible, or sit just outside of it.