This happens because the plants are continually sheared, and nothing else. No thinning cuts to let light into the center. Never a rejuvenating cut to cultivate some new canes.
All the plant's leaf surface concentrated there, on thin twigs, shading itself out, that's not a healthy thing, and not nearly as pretty as a full-to-the floor hedge can be.
If the hedge were ours to maintain we'd start with a later winter/early spring cut (before budbreak) to 1/2 or 1/3 their present height, with a few canes on each plant cut further down. New growth would come from the base on up and by July we'd be nipping it to level it out and make it branch a bit.